READING THE SIGNATURES OF CHANGING … · L’analisi effettuata sulle carote lagunari implementa...

Università degli Studi di Padova

Dipartimento di Geoscienze

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SCUOLA DI DOTTORATO IN SCIENZE DELLA TERRA

CICLO XXVIII

READING THE SIGNATURES OF

CHANGING ENVIRONMENTAL FORCINGS

IN SALT-MARSH BIOGEOMORPHIC SYSTEMS

Direttore della Scuola: Prof. Fabrizio Nestola

Supervisore: Prof. Andrea D’Alpaos

Co-supervisore: Prof. Massimiliano Ghinassi

Dottorando: Marcella Roner

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TABLE OF CONTENTS

ABSTRACT ...................................................................................................................... 5

RIASSUNTO ................................................................................................................... 7

CHAPTER 1:

INTRODUCTION .......................................................................................................... 9

1.1 OVERVIEW ....................................................................................................... 9

1.2 STATE OF THE ART ....................................................................................... 9

1.2.1 Lagoons and salt marshes ................................................................... 9

1.2.2 Biomorphodynamic models of salt-marsh evolution ..................... 15

1.2.3 The Venice Lagoon .............................................................................. 17

1.3 GOALS OF THE STUDY ................................................................................. 19

1.4 THESIS OUTLINE ............................................................................................ 20

CHAPTER 2:

SPATIAL VARIATION OF SALT-MARSH ORGANIC AND INORGANIC

DEPOSITION AND ORGANIC CARBON ACCUMULATION: INFERENCES

FROM THE VENICE LAGOON, ITALY ................................................................... 21

2.1 OVERVIEW ....................................................................................................... 21

2.2 PAPER ................................................................................................................ 21

2.2.1 Abstract ................................................................................................. 22

2.2.2 Introduction .......................................................................................... 23

2.2.3 Study area ............................................................................................. 27

2.2.4 Materials and methods ....................................................................... 28

2.2.4.1 Determination of the organic fraction: three different

analyses ................................................................................................. 29

2.2.4.2 H2O2 and NaClO treatments .................................................. 31

2.2.4.3 Loss On Ignition ...................................................................... 31

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2.2.4.4 Particle size analysis ............................................................... 32

2.2.4.5 Above-ground biomass .......................................................... 33

2.2.5 Results ................................................................................................... 34

2.2.5.1 Elevation ................................................................................... 34

2.2.5.2 Sediment dry bulk density, inorganic sediment density

and grain size ....................................................................................... 34

2.2.5.3 Soil Organic Matter ................................................................. 39

2.2.5.4 Above-ground biomass .......................................................... 41

2.2.5.5 Soil Organic Carbon ............................................................... 42

2.2.6 Discussion ............................................................................................. 43

2.2.7 Conclusions .......................................................................................... 47

CHAPTER 3:

LATEST HOLOCENE DEPOSITIONAL HISTORY OF THE SOUTHERN

VENICE LAGOON (ITALY) ........................................................................................ 51

3.1 OVERVIEW ...................................................................................................... 51

3.2 PAPER ............................................................................................................... 51

3.2.1 Abstract ................................................................................................. 52

3.2.2 Introduction ......................................................................................... 53

3.2.3 Geological setting ................................................................................ 56

3.2.4 Methods ................................................................................................ 58

3.2.4.1 Sedimentological analysis ...................................................... 60

3.2.4.2 Geochronological analyses .................................................... 60

3.2.4.2.1 Radiocarbon analyses ................................................ 60

3.2.4.2.2 Radionuclides 210Pb and 137Cs .................................... 61

3.2.4.3 Accretion model ...................................................................... 63

3.2.5 Results ................................................................................................... 63

3.2.5.1 The study deposits .................................................................. 64

3.2.5.1.1 Sedimentology ............................................................ 64

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3.2.5.1.2 Stratigraphy ................................................................. 68

3.2.5.2 Radiocarbon datings ............................................................... 69

3.2.5.3 Radionuclides 210Pb and 137Cs ................................................. 71

3.2.5.4 Accretion model ...................................................................... 71

3.2.6 Discussion ............................................................................................. 75

3.2.6.1 Depositional history of the Punta Cane area ....................... 75

3.2.6.2 Depositional history of the area landward of Punta

Cane ....................................................................................................... 77

3.2.6.3 Depositional history of the area seaward of Punta Cane .. 79

3.2.7 Conclusions ........................................................................................... 80

CHAPTER 4:

DYNAMICS OF SALT-MARSH LANDSCAPES UNDER HIGH SEDIMENT

DELIVERY RATES: THE CASE OF THE SOUTHERN VENICE LAGOON ..... 83

4.1 OVERVIEW ....................................................................................................... 83

4.2 INTRODUCTION ............................................................................................ 83

4.3 GEOLOGICAL AND GEOMORPHOLOGICAL SETTING ...................... 88

4.3.1 The southern Venice Lagoon ............................................................. 88

4.3.2 The Brenta River .................................................................................. 89

4.3.3 The study site ....................................................................................... 93

4.4 MATERIALS AND METHODS ..................................................................... 94

4.4.1 Sedimentological analysis .................................................................. 95

4.4.2 Determination of the organic fraction .............................................. 95

4.4.3 Particle size analysis ............................................................................ 95

4.4.4 X-Ray Fluorescence .............................................................................. 96

4.5 RESULTS ........................................................................................................... 97

4.5.1 Sedimentological analysis .................................................................. 97

4.5.2 The organic fraction ............................................................................. 100

4.5.3 Particle size analysis ............................................................................ 101

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4.5.4 X-Ray Fluorescence ............................................................................. 102

4.6 DISCUSSION .................................................................................................... 103

4.7 CONCLUSIONS ............................................................................................... 108

CHAPTER 5:

CONCLUSIONS ............................................................................................................. 111

REFERENCES ................................................................................................................. 115

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ABSTRACT

The question on whether actual tidal morphologies are in equilibrium with

current environmental conditions or retain signatures of past climatic changes or

human interventions is a classical and fascinating one, furthermore being of

intellectual as well as practical interest. Understanding the dynamic response of

tidal landscapes to past conditions is critical to predict their response to future

environmental changes, such as rate of relative sea-level rise and sediment supply.

This is an open and fundamentally important point, particularly in times of

natural and anthropogenic changes, during which tidal environments are most

exposed to possibly irreversible transformations with far-reaching socio-economic

and ecological implications worldwide.

The proposed work aims at analyzing the signatures of changing

environmental forcings imprinted in the landscape and in the sedimentary record

of the Venice Lagoon to refine our knowledge of tidal landforms dynamics. The

thesis is developed following a biogeomorphic approach to the study of salt-

marsh landscapes. Marsh biomorphological evolution, in response to changes in

the environmental forcings, is analyzed investigating the relative role and mutual

interactions and adjustments between physical and biological processes shaping

the salt-marsh landscape.

This thesis was carried out through a series of extensive temporal and

spatial high-resolution morphological, sedimentological, geochronological and

elemental analyses, aimed at exploring the main features of sub-surface marsh

samples and lagoonal sediment cores.

The study of sub-surface marsh samples highlights the mutual role of

inorganic and organic accretion on salt marshes, which is mainly driven by the

inorganic component near the channels, while the organic component largely

contributes in the inner-marsh portion. The analyses carried out on sediment cores

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refine the knowledge of the latest Holocene sedimentary succession of the Venice

Lagoon, and furnish a chronostratigraphical model for the evolution over the last

two millennia. In particular, for a salt-marsh succession, the analyses highlight the

occurrence of a delayed marsh-dynamic response to changing sediment delivery

rates.

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RIASSUNTO

La questione inerente l’equilibrio delle morfologie tidali con le attuali

condizioni ambientali, o se esse conservino tutt’ora i segni dei cambiamenti

climatici o degli interventi antropici passati, è un argomento classico ed

affascinante nel campo delle Geoscienze, oltre ad essere di interesse sia

intellettuale che pratico. Comprendere i meccanismi che governano la risposta di

un ambiente a marea a variazioni passate delle forzanti ambientali è fondamentale

per prevedere la loro risposta a cambiamenti ambientali futuri, quali il tasso di

innalzamento del livello del mare relativo e l’apporto di sedimenti. Si tratta di un

tema tutt’oggi sospeso e di fondamentale importanza, soprattutto in tempi di

cambiamenti sia naturali che umanamente indotti, durante i quali gli ambienti

tidali sono maggiormente esposti a trasformazioni potenzialmente irreversibili,

con implicazioni di vasta portata socio-economica ed ecologica in tutto il mondo.

Il presente lavoro si propone di analizzare le firme del cambiamento delle

forzanti ambientali impresse nella morfologia e nel record sedimentario della

Laguna di Venezia, con lo scopo di affinare la conoscenza delle dinamiche tidali.

La tesi volge allo studio di sistemi di barena attraverso un approccio

biogeomorfologico. L’evoluzione geomorfologica delle barene, in risposta ai

cambiamenti delle forzanti ambientali, è analizzata investigando il ruolo relativo,

le interazioni reciproche e le regolazioni esistenti tra i processi fisici e biologici che

modellano gli ambienti di barena.

Il lavoro è realizzato attraverso una serie di analisi morfologiche,

sedimentologiche, geocronologiche ed elementali, eseguite ad alta risoluzione

spazio-temporale, volte ad esplorare le principali caratteristiche sia di campioni

sub-superficiali di barena, sia di carote di sedimenti lagunari.

Lo studio dei campioni sub-superficiali evidenzia il ruolo reciproco delle

componenti organica ed inorganica nell’accrezione delle barene, la quale è

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principalmente guidata dalla componente inorganica in prossimità dei canali,

mentre la componente organica contribuisce in gran parte nelle porzioni più

interne delle barene. L’analisi effettuata sulle carote lagunari implementa la

conoscenza della successione sedimentaria tardo-Olocenica della Laguna di

Venezia, e fornisce un modello di evoluzione cronostratigrafica degli ultimi due

millenni. In particolare, le analisi effettuate su una successione sedimentaria di

barena, evidenziano la presenza di una risposta dinamica ritardata dell’ambiente a

cambiamenti nei tassi di apporto sedimentario.

Introduction

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CHAPTER 1

INTRODUCTION

1.1 OVERVIEW

This study deals with lagoonal deposits accumulated in the Venice Lagoon

(Italy) during the last millennium and aims at defining the main salt-marsh

modifications in response to changes in both natural (e.g., Relative Sea Level Rise,

hereinafter RSLR) and anthropogenic (e.g., sediment supply) forcings. Two key-

areas were investigated to detect the mutual interactions and adjustment between

physical and biological processes and their effects on salt-marsh surfaces, and to

identify the effects on salt-marsh evolution of changing rates of RLSR and

sediment supply enhanced by river diversions.

1.2 STATE OF THE ART

1.2.1 Lagoons and salt marshes

A lagoon is “a coastal basin dominated by tides, separated from the sea by a

sandbar but communicating with it through lagoon inlets” (Brambati, 1988).

Lagoons exhibit particular physical and biological characteristics, are

characterized by extremely high biodiversity and elevated rates of primary

productivity, host typical ecosystems and landforms and important socio-

economic activities worldwide (e.g., Cronk and Fennessy, 2001; Barbier et al.,

2011).

Referring to the vertical position within the tidal frame, supratidal areas are

permanently dry, being located above the Maximum High Water Level (MHWL),

whereas subtidal areas are always submerged, lying below the Minimum Low

Introduction

10

Water Level (MLWL) (Fig. 1.1). Intertidal areas are instead subjected to tidal

fluctuations, their elevation ranging between MHWL and MLWL. From a

morphological point of view, salt marshes, tidal flats and subtidal platforms

represent the three major unchanneled lagoon sub-environments. Salt marshes are

topographically the highest, with elevation between Mean Sea Level (MSL) and

MHWL. They are populated by a luxuriant halophytic vegetation and display a

generally flat weakly concave-up profile (Adam, 1990). Tidal flats are

characterized by elevations between MSL and MLWL, they lack any halophytic

vegetation but can be populated by seagrass meadows, and are fully exposed to

the atmosphere only during exceptionally low tides. Subtidal platforms are

located below MLWL, and are therefore perennially submerged (e.g., Allen, 2000).

Fig. 1.1. Schematic representation of the subdivision between different sub-environments in the

vertical tidal frame.

The unchanneled portions of the tidal landscape are tightly intertwined

functionally with the network of channels which cuts through them and exerts a

fundamental control on hydrodynamic, sediment and nutrient exchanges within

tidal environments (e.g., D’Alpaos et al., 2005) (Fig. 1.2).

Introduction

11

Fig. 1.2. Examples of Lagoon sub-environments: (A) Salt marshes; (B) Tidal flats; (C) Subtidal

platforms; (D) Network of channels.

As part of the coastal zone, salt marshes can be defined as “vegetated areas

located between coastal hinterlands and daily flooded coastal areas” (Bartholdy,

2012). They thus, represent a transition zone between submerged and emerged

environments, occupying the upper margins of the intertidal landscape. They

form in low-energy, wave-protected shorelines and are characterized by extremely

high primary and secondary production (e.g., Mitsch and Gosselink, 2000; Cronk

and Fennessy, 2001; Barbier et al., 2011). Salt marshes exist in all climate zones,

from the tropics to high-arctic coastal environments (Bartholdy, 2012), providing a

high number of benefits and ecosystem services (e.g., Costanza et al., 1997) to

human and, in general, to the entire environment, due to their unique position in

the tidal frame. For millennia, humans have relied on marshes for direct

provisioning of raw material and food (Davy et al., 2009). Although harvesting of

marsh grasses and use of salt marshes as pasture land has decreased nowadays,

these services are still important locally in both developed and developing areas of

the world (Gedan et al., 2009). Salt marshes provide coastal protection from waves

Introduction

12

and storm surges, as well from coastal erosion (e.g., Möller et al., 1999; Davy et al.,

2009; Howes et al, 2010; Temmerman et al., 2013; Möller et al., 2014). They reduce

impacts of incoming waves by reducing their velocities, height and duration

(Gedan et al., 2011; Möller et al., 2014), and they reduce storm surge duration and

height by providing extra water uptake and holding capacity in comparison to the

sediments of unvegetated tidal flats (Costanza et al., 2008). Salt marshes act as

natural filters that purify water from nutrient and pollutants (Mitsch and

Gosselink, 2000; Costanza et al, 2007; Larsen et al., 2010;) because as water passes

through marshes, it slows due to the baffling and friction effect of marsh

vegetation (Morgan et al., 2009). Suspended sediments are then deposited on the

marsh surface, facilitating nutrient uptake by vegetation. Salt-marsh ecosystem

also provide nursery areas for coastal biota (e.g., Perillo et al., 2009; Silliman et al.,

2009) and serve to maintain fisheries by boosting the production of economically

and ecologically important fishery species, such as shrimp, oyster, clams and

fishes (Boesch and Turner, 1984; MacKenzie and Dionne, 2008). Because of their

complex and tightly packed plant structure, marshes provide habitat that is mostly

inaccessible to large fishes, thus providing protection and shelter for the increased

growth and survival of young fishes, shrimps and shellfish (Boesch and Turner,

1984). As one of the most productive ecosystem in the world, salt marshes serve as

an important carbon sink due to their great ability to sequester atmospheric

carbon (e.g., Chmura et al., 2003; Duarte et al., 2005; Mcleod et al., 2011; Murray et

al., 2011; Sifleet et al., 2011; Kirwan and Mudd, 2012, Ratliff et al., 2015). The

sequestered carbon by salt marshes is estimated at millions of tons of carbon

annually (Mitsch and Gosselink, 2000), which is stocked in the biomass and in

sediments (Duarte et al., 2005; Mcleod et al., 2011; Murray et al., 2011; Sifleet et al.,

2011; Duarte et al., 2013). Because of the hypoxic marsh soil conditions, the carbon

is buried and preserved for a long time, significantly contributing to the

sequestration of “blue carbon” (Nellemann et al., 2009; Mcleod et al., 2011). Blue

carbon is sequestered over the short (decennial) time scales in biomass, and over

Introduction

13

longer (millennial) time scales in marsh sediments (Duarte et al., 2005), as marshes

accrete vertically, leading to its presence both in the short-term and in the long-

term carbon cycle. This capability is unique among many of the world ecosystems,

where carbon is mostly turned over quickly and does not often move into the long

term carbon cycle (Mitsch and Gosselink, 2000; Mayor and Hicks, 2009). Lastly,

salt marshes provide important habitat for many other beneficial species, as birds,

and are important for tourism, recreation, education and research (Barbier et al.,

2011).

The future of these valuable coastal landforms and ecosystems is today at

risk, exposed as they are to possibly irreversible transformations due to the effects

of climate changes and human interferences (e.g., Delaune and Pezeshki, 2003;

Marani et al., 2007; Day et al, 2009; Kirwan et al., 2010; Mudd, 2011; Murray et al.,

2011; D’Alpaos et al., 2012; Ratliff et al., 2015). Current human threats to salt

marshes include biological invasion, eutrophication, climate change and sea level

rise, increasing air and sea surface temperatures, increasing CO2 concentrations,

altered hydrologic regimes, marsh reclamation, vegetation disturbance, and

pollution (e.g., Silliman et al., 2009; Barbier et al., 2011).

Approximatively 50% of the original salt marsh ecosystem have been

degraded or lost globally (Barbier et al., 2011), and in some areas, such as the West

Coast of the US, the loss is >90% (Bromberg and Silliman, 2009; Gedan et al., 2009).

However, the extent of salt marshes has dramatically decreased worldwide in the

last century (Day et al., 2000; Marani et al., 2003, 2007; Carniello et al., 2009; Day et

al., 2009; Gedan et al., 2009; Mcleod et al., 2011) together with their fundamental

ecosystem services. For example, salt-marsh areas in the Venice lagoon decreased

from about 180 km2 in 1811 to about 50 km2 in 2002, a reduction of more than 70%

(Marani et al., 2003, 2007; Carniello et al., 2009; D’Alpaos, 2010a). Losses of vast

amounts of marsh areas have also been documented for the San Francisco Bay

(about 200 km2, Gedan et al., 2009) over the last century, while vast amounts of

Introduction

14

wetlands disappeared in the Mississippi Delta plain (about 4800 km2, Day et al.,

2000, 2009).

The rising sea level and the lack of available sediments are key factors in

determining the drowning and disappearance of salt marshes worldwide (Day et

al., 2000; Morris et al., 2002; Reed, 2002; Marani et al., 2007; Gedan et al., 2009;

Kirwan et al., 2010; D’Alpaos et al., 2011; Mudd, 2011; D’Alpaos and Marani,

2015). In the horizontal plane the prevalence of wind-wave induced lateral erosion

over marsh progradation is responsible for the retreat of salt marsh edges

(Carniello et al., 2009; Mariotti and Fagherazzi, 2010; Marani et al., 2011; Mariotti

and Carr, 2014). Once the marsh drowns or is laterally eroded and converted to

tidal flat or to a subtidal platform.

Salt marshes are populated by halophytic vegetation species, adapted to

saline environments (Fig. 1.3). The development of vegetation over salt marshes is

mainly determined by the frequency and duration of marsh flooding (e.g., Morris

et al., 2002; Silvestri et al., 2005; Kirwan and Guntenspergen, 2012), which, in turn,

depend on elevation, position and local topography of the marshes.

Fig. 1.3. Examples of halophytic species populating the salt marshes of the Venice Lagoon. (A)

Limonium narbonense. (B) Salicornia veneta. (C) Inula crithmoides. (D) Spartina maritima. (E)

Sarcocornia fruticosa.

Introduction

15

Halophytic vegetation species which populate marsh platforms control

sediment trapping efficiency, by enhancing particle settling via reduction of

turbulence kinetic energy (e.g., Leonard and Croft, 2006; Mudd et al., 2010), by

directly capturing sediment particles (e.g., Leonard and Luther, 1995; Li and Yang,

2009), and determine vertical organic accretion, by directly depositing organic

matter due to root growth and litter deposition (Nyman et al., 2006; Neubauer,

2008; Mudd et al., 2009). Halophytes interact with inorganic sedimentation and the

rate of Relative Sea Level Rise (RSLR, sea level variations plus local subsidence),

their combined effects controlling marsh surface elevation. Surface elevation, in

turn, affects the vegetation productivity (e.g., Morris et al., 2002) which influences

inorganic sediment deposition and control organic accretion, thus closing the bio-

geomorphic feedback.

1.2.2 Biomorphodynamic models of salt-marsh evolution

The development of salt marshes occurs through the sedimentation

furnished by both inorganic and organic components, which interact with each

other and the rate of sea level. The strong dynamic coupling of biotic and abiotic

processes in salt-marsh landscapes gives rise to beautiful and complex biological

and morphological patterns, whose non-linear dynamics is a fascinating examples

of biomorphodynamics (e.g., Murray et al., 2008). The collective temporal

evolution emerging from the mutual interactions and adjustments among

hydrodynamic, morphological and biological processes (D’Alpaos et al., 2009).

Stemming from the pioneering work by Viles (1988), the long-standing paradigm

of physical processes shaping the landscape and dictating the constraints for

biological agents, forced to live within those constraints, is being abandoned, in

favor of a new perspective in which biota feeds back on, directly modify, and

contributes to shape their physical environment (e.g., Jones et al., 1994; Hupp et

al., 1995; Dietrich and Perron, 2006; Murray et al., 2008; D’Odorico et al., 2010;

Introduction

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Reinhardt et al., 2010). As for many other fields of science, the

biogeomorphological approach has become the new paradigm for studying the

form and evolution of the salt-marsh geomorphological and ecosystem structures.

As a consequence, in the last few decades a number of studies have analysed salt-

marsh biomorphological evolution (e.g., Morris et al., 2002; Mudd et al., 2004;

D’Alpaos et al., 2007; Kirwan & Murray, 2007; Marani et al., 2007; Temmerman et

al., 2007; Mudd et al., 2009, 2010; D’Alpaos, 2011; D’Alpaos et al., 2012; Fagherazzi

et al., 2012; Marani et al., 2013), although a predictive understanding of the two-

way feedbacks between physical and biological processes still appears to be

elusive. Moreover, analyses of bio-geomorphic feedbacks based on data collected

in the field at the marsh scale are rare (Mendelsshon et al., 1981; Day et al., 1998a,

1998b,1999; Morris et al, 2002; Delaune and Pezeshki, 2003; Nyman et al., 2006;

Bellucci et al., 2007; Neubauer, 2008), leading to a lack in the characterization of

spatial variations in the accretion rates, and, in particular, in the organic soil

production.

In addition, the improvement of current understanding of the processes

controlling the response and the evolution of salt marshes to changes in the

environmental forcings is still an open and fundamentally important issue. A

number of models of morphodynamic evolution have been developed (e.g., Allen,

2000; Fagherazzi et al., 2012), which offer a valuable tool to address prediction on

the fate of tidal landforms. However, the capability of existing models to provide a

comprehensive and predictive theory of tidal-landscape morphodynamic

evolution is challenged by the incomplete understanding of the many linkages

between the relevant ecological and geomorphological processes (e.g., Murray et

al. 2008; Reinhardt et al., 2010). Moreover, in many cases, morphodynamic models

resort to the common approximation of a landscape in equilibrium with current

forcings, and address predictions on future scenarios using the present observed

morphologies as an initial conditions (e.g., Kirwan et al., 2010). For salt-marsh

systems, it has recently been observed that marshes might not have yet fully

Introduction

17

respond to sea level historical acceleration, and that these tidal morphological

features might be out of equilibrium with modern rates of RSLR (Kirwan and

Murray, 2005, 2008). Moreover, marshes might not have yet fully responded also

to historical decrease in sediment supply inputs and further adjustments in the

form of decreasing marsh-platform elevation could be expected (D’Alpaos et al.,

2011).

1.2.3 The Venice Lagoon

The Venice Lagoon represents an outstanding example of man-landscape

co-existence (Gatto and Carbognin, 1981).

The Venice Lagoon formed over the last 6’000 – 7’000 years as a

consequence of the flooding of the upper Adriatic plain which followed the Würm

glaciation (Gatto and Carbognin, 1981). The first relevant human settlements

within the lagoon dated back to the 5th century AD (e.g., Dorigo, 1983; Ninfo et al.,

2009), but a severe human influence started around the 11th century AD and was

mainly aimed at hindering the shoaling of the lagoonal bottom and extend the use

of navigation channels, causing significant changes in the amount of freshwater

and sediment input into the lagoon (e.g., D’Alpaos, 2010a). Thus, as many other

tidal landscapes worldwide, the natural environment of the Venice Lagoon has

been involved in a drastic hydro-morphodynamic change, which is manifested in

the decrease of salt-marsh areas, together with a general expansion and deepening

of tidal flats and subtidal platforms over the last centuries (Marani et al., 2003,

2007; Carniello et al., 2009; D’Alpaos 2010a; D’Alpaos et al., 2013).

The Venice Lagoon is about 50 km long and 8 – 14 km wide (Fig. 1.4). It is

worth noting that some areas in the northern part of the lagoon represent the

lagoonal portion most naturally preserved and, consequently, a large number of

studies have been performed in the last decades leading to a large quantity of

available data (e.g., Brivio and Zilioli, 1996; Day et al., 1998a, 1999; Marani et al.,

Introduction

18

Fig. 1.4. Color-coded bathymetry of the Venice Lagoon.

2003; Silvestri et al., 2003; Silvestri and Marani, 2004; Sfriso et al., 2005; Silvestri et

al., 2005; Bonardi et al., 2006; Marani et al., 2006; McClennen et al., 2006; Bellucci et

al., 2007; D’Alpaos et al., 2007; Marani et al., 2007; Cola et al., 2008; Rizzetto and

Tosi, 2011; Marani et al., 2013; Boaga et al., 2014). On the contrary, the sedimentary

succession of southern portion of the Venice Lagoon is still relatively unexplored

Introduction

19

(Day et al., 1998a, 1998b, 1999; Lucchini et al., 2001; Bellucci et al., 2007; Brancolini

et al., 2008; Zecchin et al., 2008, 2009; Tosi et al., 2009a; Zecchin et al., 2011), and it

has been subjected to severe anthropogenic interferences. The main changes

occurred during the past 1,000 years and were mainly related to variations in

freshwater and sediment input due to repeated re-direction of the Brenta River

system. Engineering interventions on the southern Venice Lagoon and the related

mainland are exhaustively recorded from historical sources (e.g., topographic

maps), whereas their effects, along with their interaction with natural

aggravations, are documented in the stratigraphic record, which is considerably

expanded given the extraordinary aggradation rate of the southern Venice Lagoon

which ranges between 0.03 cm yr-1 and 1.5 cm yr-1 (Bellucci et al., 2007), up to 2.32

cm yr-1 (Day et al., 1998b).

1.3 GOALS OF THE STUDY

The lack of a comprehensive and detailed knowledge on the

biomorphodynamic evolution of salt-marsh systems raises queries concerning the

processes which control their development over time.

It is therefore of fundamental theoretical and practical importance to

improve our understanding of the processes controlling: i) the evolution of salt-

marsh bio-geomorphological patterns, and the relative importance of physical and

biological processes; ii) the response of salt-marsh systems to changing

environmental forcings.

The present work contributes to unravelling these issues by i) analyzing

modern sub-surface marsh sediments from the San Felice and Rigà salt marshes

(northern Venice Lagoon) to unravel the role of the organic and inorganic

components for marshes accretion; ii) analyzing the latest Holocene succession in

the Punta Cane area (southern Venice Lagoon) to provide evidences of salt-marsh

Introduction

20

system dynamic response to changes in sediment supply (and rates of RLSR)

provided by the Brenta River during the last millennium.

1.4 THESIS OUTLINE

The present work is presented through five chapters.

Chapter 2 deals with the analysis of the variations of organic and inorganic

deposition on salt marshes to test the role and the effect of the organic and

inorganic components for marshes accretion and bio-geomorphological evolution.

The results are organized in a journal paper accepted for publication: M. Roner et

al., Spatial variation of salt-marsh organic and inorganic deposition and organic carbon

accumulation: Inferences from the Venice Lagoon, Italy, Advances in Water Resources

(2015), http://dx.doi.org/10.1016/j.advwatres.2015.11.011.

Chapter 3 and Chapter 4 present the results obtained by a study on the

latest Holocene depositional history of salt-marsh, tidal-flat, subtidal-platform

systems in the southern Venice Lagoon, in the Punta Cane area. In Chapter 3 the

results of sedimentological and chronological analyses are discussed, and a

detailed age model for the area over the last two millennia is furnished. Chapter 4

examines in depth the sedimentological features of the salt-marsh succession to

analyze and discuss the evidence of a high delivery rate provided by the Brenta

River system. Through the employment of a multidisciplinary approach, the

results highlight the occurrence of a dynamic response of the salt-marsh system to

changes in the forcings. Chapters 3 and 4 provide material for two further

publications, which are currently in preparation for submission.

Chapter 5 summarizes the main results of this thesis.

http://dx.doi.org/10.1016/j.advwatres.2015.11.011

Salt-marsh physical and biological processes

21

CHAPTER 2

SPATIAL VARIATION OF SALT-MARSH ORGANIC

AND INORGANIC DEPOSITION AND ORGANIC

CARBON ACCUMULATION: INFERENCES FROM THE

VENICE LAGOON, ITALY.

2.1 OVERVIEW

This chapter is a journal paper accepted for publication in Advances in

Water Resources (http://dx.doi.org/10.1016/j.advwatres.2015.11.011). The

manuscript aims at analyzing the variations of organic and inorganic deposition

on salt marshes considering a linear spatial variability, to unravel the role of the

organic and inorganic components for marshes accretion. Moreover, three

different methods are examined for the determination of the organic content, and

an estimate of the soil organic carbon for two marshes in the northern Venice

Lagoon is provided.

2.2 PAPER

MARCELLA RONER1, ANDREA D’ALPAOS1, MASSIMILIANO

GHINASSI1, MARCO MARANI2,3, SONIA SILVESTRI3, ERICA

FRANCESCHINIS4, NICOLA REALDON4

1Dept. of Geosciences, University of Padova, via Gradenigo 6, 35131

Padova, Italy.

2Dept. of Civil, Environmental and Architectural Engineering (ICEA),

University of Padova, via Marzolo 9, 35131 Padova, Italy.


22

3Division of Earth and Ocean Sciences, Nicholas School of the Environment

and Department of Civil and Environmental Engineering, Pratt School of

Engineering, Duke University, Durham, North Carolina 27708, USA.

4Dept. of Pharmaceutical and Pharmacological Sciences, University of

Padova, via Marzolo 5, 35131 Padova, Italy.

2.2.1 Abstract

Salt marshes are ubiquitous features of the tidal landscape governed by

mutual feedbacks among processes of physical and biological nature. Improving

our understanding of these feedbacks and of their effects on tidal

geomorphological and ecological dynamics is a critical step to address issues

related to salt-marsh conservation and response to changes in the environmental

forcing. In particular, the spatial variation of organic and inorganic soil production

processes at the marsh scale, a key piece of information to understand marsh

responses to a changing climate, remains virtually unexplored. In order to

characterize the relative importance of organic vs. inorganic deposition as a

function of space, we collected 33 shallow soil sediment samples along three

transects in the San Felice and Rigà salt marshes located in the Venice lagoon,

Italy. The amount of organic matter in each sample was evaluated using Loss On

Ignition (LOI), a hydrogen peroxide (H2O2) treatment, and a sodium hypochlorite

(NaClO) treatment following the H2O2 treatment. The grain size distribution of the

inorganic fraction was determined using laser diffraction techniques. Our study

marshes exhibit a weakly concave-up profile, with maximum elevations and

coarser inorganic grains along their edges. The amount of organic and inorganic

matter content in the samples varies with the distance from the marsh edge and is

very sensitive to the specific analysis method adopted. The use of a H2O2 + NaClO

treatment yields an organic matter density value which is more than double the

value obtained from LOI. Overall, inorganic contributions to soil formation are


23

greatest near the marsh edges, whereas organic soil production is the main

contributor to soil accretion in the inner marsh. We interpret this pattern by

considering that while plant biomass productivity is generally lower in the inner

part of the marsh, organic soil decomposition rates are highest in the better

aerated edge soils. Hence the higher inorganic soil content near the edge is due to

the preferential deposition of inorganic sediment from the adjacent creek, and to

the rapid decomposition of the relatively large biomass production. The higher

organic matter content in the inner part of the marsh results from the small

amounts of suspended sediment that makes it to the inner marsh, and to the low

decomposition rate which more than compensates for the lower biomass

productivity in the low-lying inner zones. Finally, the average soil organic carbon

density from the LOI measurements is estimated to be 0.044 g C cm-3. The

corresponding average carbon accumulation rate for the San Felice and Rigà salt

marshes, 132 g C m-2 yr-1, highlights the considerable carbon stock and

sequestration rate associated with coastal salt marshes.

Keywords: tidal environments, salt marshes, soil organic matter, blue carbon.

2.2.2 Introduction

Coastal salt marshes represent a transition zone between submerged and

emerged environments, occupying the upper margins of the intertidal landscape.

Because of their unique position in the tidal frame, salt marshes represent a

crucially important ecosystem. They offer valuable services, by providing a buffer

against wave and storm surges (e.g., Möller et al., 1999; Howes et al, 2010;

Temmerman et al., 2013; Möller et al., 2014), nursery areas for coastal biota (e.g.,

Perillo et al., 2009; Silliman et al., 2009) and filtering of nutrients and pollutants

(e.g., Costanza et al, 1997; Larsen et al., 2010). In the last few decades a number of


24

authors also highlighted the importance of salt marshes serving as an organic

carbon sink due to their great ability to sequester atmospheric carbon (e.g.,

Chmura et al., 2003; Mcleod et al., 2011; Murray et al., 2011; Sifleet et al., 2011;

Kirwan and Mudd, 2012). The future of these valuable coastal landforms and

ecosystems is today at risk, exposed as they are to possibly irreversible

transformations due to the effects of climate changes and human interferences

(e.g., Delaune and Pezeshki, 2003; Marani et al., 2007; Day et al, 2009; Kirwan et al.,

2010; D’Alpaos et al., 2012; Murray et al., 2011; Ratliff et al., 2015). The extent of

these coastal features, in fact, has dramatically decreased worldwide in the last

century (Day et al., 2000; Marani et al., 2003, 2007; Carniello et al., 2009; Gedan et

al., 2009; Mcleod et al., 2011). For example, salt-marsh areas in the Venice lagoon

decreased from about 180 km2 in 1811 to about 50 km2 in 2002, a reduction of more

than 70% (Marani et al., 2003, 2007; Carniello et al., 2009; D’Alpaos, 2010a). Losses

of vast amounts of marsh areas have also been documented for San Francisco Bay

(about 200 km2, Gedan et al., 2009) over the last century, while vast amounts of

wetlands disappeared in the Mississippi Delta plain (about 4800 km2, Day et al.,

2000, 2009).

The rising sea level and the lack of available sediments are key factors in

determining the drowning and disappearance of salt marshes worldwide (Day et

al., 2000; Morris et al., 2002; Reed, 2002; Marani et al., 2007; Gedan et al., 2009;

Kirwan et al., 2010; D’Alpaos et al., 2011; Mudd, 2011; D’Alpaos and Marani,

2015). In the horizontal plane the prevalence of wind-wave induced lateral erosion

over marsh progradation is responsible for the retreat of salt marsh edges

(Carniello et al., 2009; Mariotti and Fagherazzi, 2010; Marani et al., 2011; Mariotti

and Carr, 2014). Once the marsh drowns or is laterally eroded and converted to

tidal flat or to a subtidal platform, unless the environmental conditions change (i.e.

the sediment concentration increases) the re-growth of the marsh platforms is

unlikely because the system is characterized by a hysteretic behavior (e.g., Kirwan

and Murray, 2007; Marani et al., 2010).


25

Salt marshes are populated by halophytic vegetation species, adapted to

saline environments. The spatial distribution of halophytes over salt marshes is

organized in characteristic patches, a phenomenon known as zonation (Chapman,

1976; Adam, 1990; Pennings et al, 2005; Silvestri et al., 2005; Marani et al., 2006;

Moffett et al., 2010). It has been recently shown that zonation patterns are not just

the result of ecological and physiological processes and that their emergence is the

consequence of the feedback on soil accretion of organic soil production by plants,

which act as a landscape engineer (Marani et al., 2013, Da Lio et al., 2013). The

development of vegetation over salt marshes is mainly determined by the

frequency and duration of marsh flooding (e.g., Morris et al., 2002; Silvestri et al.,

2005; Kirwan and Guntenspergen, 2012), which, in turn, depend on elevation,

position and local topography of the marshes. Halophytic vegetation species

which populate marsh platforms control sediment trapping efficiency, by

enhancing particle settling via reduction of turbulence kinetic energy (e.g.,

Leonard and Croft, 2006; Mudd et al., 2010), by directly capturing sediment

particles (e.g., Leonard and Luther, 1995; Li and Yang, 2009), and determine

vertical organic accretion, by directly depositing organic matter due to root

growth and litter deposition (Nyman et al., 2006; Neubauer, 2008; Mudd et al.,

2009). Halophytes interact with inorganic sedimentation and the rate of Relative

Sea Level Rise (RSLR, sea level variations plus local subsidence), their combined

effects controlling marsh surface elevation. Surface elevation, in turn, affects the

vegetation productivity (Morris et al., 2002) which influences inorganic sediment

deposition and control organic accretion, thus closing the bio-geomorphic

feedback. In this framework it is clear that the interaction between physical and

biological processes acting within salt marsh systems plays a fundamental role in

salt-marsh survival or disappearance. Although in the last few decades a number

of studies have analysed salt-marsh biomorphological evolution (e.g., Morris et al.,

2002; Mudd et al., 2004; D’Alpaos et al., 2007; Kirwan & Murray, 2007; Marani et

al., 2007; Temmerman et al., 2007; Mudd et al., 2009; D’Alpaos, 2011; D’Alpaos et


26

al., 2012; Fagherazzi et al., 2012; Marani et al., 2013), a predictive understanding of

the two-way feedbacks between physical and biological processes still appears to

be elusive. Moreover, analyses of bio-geomorphic feedbacks based on data

collected in the field at the marsh scale are rare (Mendelsshon et al., 1981; Day et

al., 1998a, 1998b, 1999; Morris et al, 2002; Delaune and Pezeshki, 2003; Nyman et

al., 2006; Bellucci et al., 2007; Neubauer, 2008). The evolution in time of marsh

elevation z (x, t) (referenced to Mean Sea Level -- hereinafter MSL) at a given site x

and at time t, is governed by the sediment continuity equation (where erosion is

neglected because of the stabilizing presence of vegetation, see Marani et al., 2010,

2013):

𝑧(𝒙,𝑡)

𝑡= 𝑄𝑖 (𝒙, 𝑡) + 𝑄𝑜(𝒙, 𝑡) − 𝑅 [1]

where 𝑄𝑖(𝒙, 𝑡) and 𝑄𝑜(𝒙, 𝑡) are the local rates of inorganic and organic deposition,

respectively, and 𝑅 is the rate of RSLR. In equilibrium conditions, the marsh

elevation referenced to MSL is constant over time and therefore the left-hand side

term in the above equation vanishes. The sediment balance equation then reads:

𝑄𝑖 (𝒙, 𝑡) + 𝑄𝑜(𝒙, 𝑡) = 𝑅 [2]

Hence, in a stable marsh, if 𝑄𝑖 increases, 𝑄𝑜 needs to decrease by the same

magnitude, and vice versa, such that the forcing rate of RSLR is matched

everywhere in the marsh. Field observations and numerical models (e.g.,

Christiansen et al., 2000; Temmerman et al., 2003; D’Alpaos et al., 2007; Kirwan et

al., 2008) suggest that marsh inorganic accretion rates, and the related platform

elevations, decrease with distance from the main channels. Therefore, the organic

accretion should be expected to gradually increase as the distance from the main

channel increases for the equilibrium assumption to hold. However, a direct and

detailed characterization of spatial variations in the accretion rates, and, in

particular, in the organic soil production, is still lacking, and is the focus of the

present work.


27

The paper is organized as follows. In the next section we provide a brief

description of our study sites within the Venice lagoon. We then fully describe the

methods used to determine the organic and inorganic sediment content, grain size

distribution, above-ground biomass. The subsequent Results and Discussion

sections analyze the role of physical and biological factors shaping the tidal

landscape, and how these factors influence salt-marsh biogeomorphic patterns.

2.2.3 Study area

The Venice lagoon (Fig. 2.1a) is part of a foreland basin located between the

NE-verging northern Appenninic chain and the SSE-verging eastern South-Alpine

chain (Italy). Located in the northwestern Adriatic Sea, the Venice lagoon is the

largest lagoon in the Mediterranean, with an area of about 550 km2, a mean water

depth of 1.5 m, and a semi-diurnal micro-tidal regime (maximum water excursion

at the inlets of ± 70 cm around MSL). The Lagoon is connected with the Adriatic

Sea via three inlets: Lido, Malamocco and Chioggia (Fig. 2.1a).

The two study areas (Figs. 2.1a, 2.1b, 2.1c) are the San Felice and the Rigà

salt marshes, located in the northern Venice lagoon, close to the Lido inlet and

adjacent to the San Felice Channel (see Marani et al., 2003, Silvestri et al. 2003,

2005, for a detailed description of the study sites from a geomorphological and

ecological perspective). These marshes, about 2 km apart, have been studied for

more than 10 years and a large amount of data is available. In addition, it is worth

noting that these marshes maintained their main characteristics because of their

location in the most naturally preserved portion of the lagoon. The San Felice and

the Rigà salt marshes are incised by meandering tidal networks and are colonized

by a wide range of halophytic vegetation species: Salicornia veneta, Spartina

maritima, Limonium narbonense, Sarcocornia fruticosa, Juncus maritimus, Inula

crithmoides, Puccinellia palustris, Halimione portulacoides, Suaeda maritima,

Arthrocnemum macrostachyum, Aster tripolium (Silvestri et al., 2003). The marsh soil


28

Fig. 2.1. (a) Location of the study sites in the northern Venice lagoon, Italy. (b) San Felice salt

marsh. (c) Rigà salt marsh.

is composed of clayey sandy silt and of a large organic fraction. Accretion rates in

the San Felice area were estimated in 3.0 mm yr-1 by Day et al. (1998a), whereas the

rate of sea-level rise is of about 2.0 mm yr-1 (Carbognin et al., 2004), and the local

subsidence is about 1.0 mm yr-1 (Carbognin et al., 2004; Strozzi et al., 2013), for a

total rate of relative sea level rise of about 3.0 mm yr-1.

2.2.4 Materials and methods

We collected 33 undisturbed cubic sediment samples with side of 5 cm,

delimited at the top by the present-day depositional interface, to study the spatial

variations of the soil inorganic and organic matter content, for the determination

of the inorganic (𝑄𝑖) and the organic (𝑄𝑜) accretion rates, respectively. Each sample

consists of massive brownish to blackish silt and contains abundant stems and

roots. Some of the samples also contained scattered mm-size bivalve and

gastropod shells and/or shell fragments. The samples were collected along three 40


29

m long transects in two salt marshes in the Northern Venice lagoon (Fig. 2.1a).

Transect 1 and 2 were located in the San Felice salt marsh, whereas Transect 3 was

located in the Rigà salt marsh (Figs. 2.1b, 2.1c). At both sites the transects started

on the marsh edge close to the main channel (the San Felice channel) and ended

close to a minor inner tidal creek (width of the order of a few tens of centimeters).

Samples were collected with a spacing of 2.5 m for the first 10 m from the San

Felice channel, whereas the spacing was 5 m for the remaining 30 m. Elevation and

geographic location of each sample were determined using two TOPCON GR-3

GPS receivers (dual frequency - L1/L2 - and dual constellation - NavStar/Glonass -

with integrated Tx/Rx UHF radio).

A volume of 8 cm3 from each sediment sample, extract at a depth of 3 cm

from the top sample surface, was dried at 60°C for 36 hours, until a constant

weight was obtained. The difference in weight between the wet and dry samples

was used to estimate the sediment dry bulk density, where the value of the total

dry mass was corrected for the presence of saline water (Dadey et al., 1992).

The amount of inorganic (𝑄𝑖) and organic fraction (𝑄𝑜), together with the

grain size and the above-ground biomass distribution, were measured and

analyzed for each sample in order to provide a complete characterization of the

processes acting on the salt-marsh surface. Finally, an analysis of the soil organic

carbon content was performed in order to estimate the carbon stock potential of

these marshes.

2.2.4.1 Determination of the organic fraction: three different analyses

Because all the methods available for the determination of soil organic

matter have their own specific limitations, different methodologies were used to

determine the organic matter content in the samples: 1) a treatment with hydrogen

peroxide (H2O2); 2) treatments with H2O2 and, in succession, sodium hypochlorite

(NaClO); 3) Loss On Ignition (LOI) analyses. It should be emphasized that the


30

chemical treatments to determine the organic matter content are not frequently

used, mainly because they are time consuming. However, they allow the

preservation of the original grain size of the inorganic fraction for grain-size

analysis, contrary to the LOI procedure, where sample crumbling and heating

modifies the size of clastic particles. The LOI approach is broadly applied in the

literature, it is faster than chemical methods and permits the simultaneous

analysis of a large number of samples. Moreover, the organic carbon content in a

soil is usually defined in the literature on the basis of LOI measurements,

providing a wider context for direct comparisons. However, an accepted

standardized LOI procedure is still lacking. The need for a standard LOI

procedure has been widely acknowledged (Heiri et al, 2001), but the scientific

community continues to use heterogeneous procedures in terms of ignition

temperatures, exposure times, sample sizes, position in the muffle furnace, etc.

(Heiry et al., 2001; Barillé-Boyer et al., 2003). The results from these different

procedures cannot be directly compared, and may underestimate or overestimate

the actual organic matter content depending on the specific sample type of organic

matter present. For example, during combustion the samples may lose structural

water in the range of temperatures from 450°C to 600°C (Ball, 1964; Howard and

Howard, 1990) in sediments characterized by high clay content, thereby

significantly affecting soil organic matter estimates (Mook and Hoskin, 1982;

Dankers and Laane, 1983; Barillé-Boyer et al., 2003). Between 425°C and 520°C a

potential loss of CO2 can occur in minerals or in sediments containing inorganic

carbon, such as siderite (FeCO3), magnesite (MgCO3), rhodocrosite (MnCO3) and

dolomite (CaMg(CO3)2) (Weliky et al., 1983; Howard and Howard, 1990;

Southerland, 1998; Santisteban et al., 2004). Overall, a LOI test at 550°C leads to a

large overestimation of the loss of organic matter mass (Frangipane et al., 2009).


31

2.2.4.2 H2O2 and NaClO treatments

The sub-samples, each characterized by a volume of 8 cm3, were treated

with 35% H2O2 for 36 hours. At the end of the oxidation, when no visible frothing

occurred, dilution with deionized water, decantation (for 24 hours), siphoning,

and drying were carried out. The weight of each sample was again measured after

drying. A further treatment with 5% NaClO was subsequently applied (for 24

hours) in order to remove the remnant organic matter, which mainly occurred

under the shape of sub-millimetric plant debris. After this second treatment, the

weight of each dried sample was measured again. The loss in weight affecting

each sample during these procedures provided an estimation of the amount of

organic matter dissolved during each treatment. The results of our analyses will be

expressed as soil organic matter density (computed as the ratio between the

amount of organic matter after each treatment and the total volume), and

inorganic sediment density (ratio between the amount of inorganic residual after

each treatment and the total volume).

2.2.4.3 Loss On Ignition

Our choice of the temperature and duration of ignition in the LOI process

was made on the basis of the existing literature (Ball, 1964; Frangipane et al., 2009;

Protocol of SFU Soil Science Lab, 2011). A total amount of 2 g of dry sediment for

each sample, dried at 60°C for 36 hours, was used to perform LOI analyses. The

sediment was crumbled in a ceramic mortar and placed in a dry ceramic crucible.

The LOI process started with a temperature increase of 5°C/min until reaching

375°C, and continued at a constant temperature for 16 hours (Ball, 1964;

Frangipane et al., 2009; Protocol of SFU Soil Science Lab, 2011). The difference in

weight, before and after the process, was used to estimate the amount of organic

matter which was combusted. As for the soil organic matter determined through


32

the chemical treatments described above, the LOI soil organic content will be

expressed as the density of soil organic matter computed from the sediment dry

bulk density at the end of the procedure (because of in the LOI analysis the

samples volume was unknown, we estimated the volume of these samples by the

ratio between the total mass of each sample and the sediment dry bulk density for

the same sample previously calculated for the chemical treatments).

The results obtained from LOI measurements were subsequently used to

determine the content of soil organic carbon in each sample according to the

relationship proposed by Craft et al. (1991):

Organic Carbon = 0.40 (LOI) + 0.0025 (LOI)2 [3]

2.2.4.4 Particle size analysis

The inorganic particle size distribution was performed after the removal of

the organic matter through the H2O2 + NaClO treatment. In samples mainly

composed by silt, where a large amount of organic matter is bound to the mineral

matrix, the removal by chemical reagents is a common pretreatment for this

analysis (Gee and Bauder, 1986; Allen and Thornley, 2004; Gray et al., 2010),

although the benefits brought by the use of chemical agents are not clear. Allen

and Thornley (2004) state that there are no advantages from the treatment with

H2O2 before laser granulometry. On the contrary, Gray et al. (2010) recommend the

employment of H2O2 treatment on samples with moderate to large amount of

organics composed of stem pieces, root fragments and seeds.

On San Felice and Rigà samples, the particle size analysis was performed on

the residual inorganic sediment fraction obtained after the H2O2 + NaClO

treatment. Deionized water was added to each sample to obtain a dispersed

particulate sample. The particle size analysis was carried out using a Mastersizer

2000 (Version 5.40, MALVERN INSTRUMENTS). The Mastersizer 2000 uses laser

diffraction to measure the size of particles, by measuring the intensity of light


33

scattered as a laser beam passes through the dispersed particulate sample. These

data are then analyzed to calculate the size of the particles that created the

scattering pattern. The grain size results will be presented as D10, D50 and D90

distribution.

2.2.4.5 Above-ground biomass

Each sampling site was centered in a 14 Megapixel resolution photograph

covering an area of about 100x80 cm. The vegetation density in each photo (a

proxy for the above-ground biomass) was evaluated through the superposition of

a regularly spaced grid. The grid contained 100 nodes, and the occurrence of plant

cover or soil was evaluated for each node, providing a percentage of vegetation

cover for different sampling site (see examples in Fig. 2.2).

Fig. 2.2. Estimation of the vegetation cover. The white circles indicate the presence of vegetation

on the grid nodes. (a) Example of the site at 2.5 m from the San Felice channel along transect 2.

(b) Example of the site at 25 m from the San Felice channel along transect 2.


34

2.2.5 Results

2.2.5.1 Elevation

GPS field surveys show that marsh surface elevations (Figs. 2.3a, 2.4a, 2.5a)

are highest near the marsh edge close to the San Felice channel levees (maximum

elevation along the transects was equal to 32.1 cm above MSL). Toward the inner

marsh, platform elevations decrease, reaching minimum values at about 30 m

from the edge, where stagnant pools exist (minimum elevation along the transects

was equal to 16.9 cm above MSL). Soil elevations at the end of the transects are

slightly higher than in the inner portion of the marsh due to the presence of the

levees of two small tidal creeks which cut through the marsh platform (see Fig.

2.1).

2.2.5.2 Sediment dry bulk density, inorganic sediment density and grain size

The average sediment dry bulk density (Figs. 2.3b, 2.4b, 2.5b) across all the

samples is 0.89 g cm-3. Dry bulk density is maximum at the edge of the marshes,

where its values range from 1.16 to 1.56 g cm-3. The rapid decrease in the bulk

density occurs in the first 5 – 7.5 m away from the main channel, where the values

range between 0.53 and 0.84 g cm-3. We also observed that, in transect 1 (Fig. 2.3b)

and 3 (Fig. 2.5b), the sediment dry bulk density rapidly decreases with distance

from the San Felice channel and it then edges up and down when moving towards

the inner portion of the marsh. On the contrary, along transect 2 (Fig. 2.4b)

sediment dry bulk density rapidly decreases with distance from the main channel,

but it tends to increase when moving towards the inner portion of the marsh

although being characterized by values smaller than on the marsh edge. In the last

5 m of the transects the sediment dry bulk density increases again, its values

ranging from 0.73 to 1.34 g cm-3.


35

Fig. 2.3. Transect 1 in the San Felice salt marsh. Spatial variations, measured along the distance

from the San Felice channel, of: (a) marsh surface elevation; (b) dry bulk density and density of

the inorganic fraction sediment; (c) grain size distribution; (d) density of the organic matter; (e)

vegetation or soil cover.


36

Fig. 2.4. Transect 2 in the San Felice salt marsh. Spatial variations, measured along the distance

from the San Felice channel, of: (a) marsh surface elevation; (b) dry bulk density and density of

the inorganic fraction sediment; (c) grain size distribution; (d) density of the organic matter; (e)



37

Fig. 2 5. Transect 3 in the Rigà salt marsh. Spatial variations, measured along the distance from

the San Felice channel, of: (a) marsh surface elevation; (b) dry bulk density and density of the

inorganic fraction sediment; (c) grain size distribution; (d) density of the organic matter; (e)



38

The amount of inorganic sediment, calculated for each sample by

subtraction from the organic content through the three different methods, is

reported as inorganic sediment density (Figs. 2.3b, 2.4b, 2.5b). The results show

that its trend along each transect, regardless of the method used, follows a trend

analogous to that displayed by the sediment dry bulk density along the same

transect. The average value along the transects obtained by LOI, H2O2 and H2O2 +

NaClO is equal to 0.78, 0.70 and 0.60 g cm-3, respectively.

The Mastersizer recorded a range of grain size values for the inorganic

sediment fraction comprises between 831.76 µm (maximum value) and 0.63 µm

(minimum value). On average, the grain size shows a variable distribution

between a medium sand (average coarser value: 400 µm) and a clay (average finer

value: 0.63 µm). The D10 distribution (Figs. 2.3c, 2.4c, 2.5c) is almost constant along

the three transects and ranges between 3 µm toward the inner portion of the

marshes, and 6÷7.6 µm on the marsh edges. As to the distribution of the median

grain size, D50 (Figs. 2.3c, 2.4c, 2.5c), we observe coarser grains along the marsh

edges (i.e. along the tidal channel levees) with a median grain size D50 in the range

33÷38 µm. The median grain size then becomes finer toward the inner marsh with

a median grain size in the range 12÷24 µm. The decrease in grain size with

distance from the San Felice channel reveals to be quite rapid and occurs in the

first 5 – 10 m from the marsh edge. In the remaining portion of the transect the

values of the D50 remain rather stable. On the San Felice salt marsh, from x=10 m

up to the end of the transects, values of the D50 are lower (12÷17 µm) than those

which characterize the Rigà salt marsh (15÷24 µm). The distribution of the D90

emphasizes the presence of coarser grains along the marsh edges. In the San Felice

salt marsh the decrease in D90 occurs rapidly, in the first 5 m from the San Felice

channel, from values of 103÷106 µm (on the marsh edge) to 45÷48 µm. From 5 m to

the end of these two transects the values fluctuate weakly up and down, with

values ranging from 37 µm to 55 µm. On the Rigà salt marsh the decrease in the

D90 values occurs slowly in the first 10 m from the San Felice channel, from 106 µm


39

on the marsh edge to 60 µm. From 10 to 20 m away from the channel the values

are stable, and tend to fluctuate up and down over the last 10 m of the transect,

with values between 72 µm and 42 µm.

2.2.5.3 Soil Organic Matter

The values of soil organic matter density (Figs. 2.3d, 2.4d, 2.5d) obtained by

using the three methods described above show important differences for each

sample and the trends along the transects follow a non-monotonically increase or

a decrease.

Results obtained on the basis of LOI suggest small variations in soil organic

matter density along the transects, except for the samples collected on the marsh

edges where organic density values are lower and tend to weakly increase in the

first 2.5 – 5 m from the marsh edge. In the remaining portion of the transects,

values of organic density remain almost stable, with some small peaks between 15

and 30 m away from the main channel. The average organic matter density value

along the transects obtained by LOI is equal to 0.11 g cm-3.

Results obtained through H2O2 treatments show values higher than those

obtained with the LOI method, for all samples, with an average value over the

three transects of 0.18 g cm-3. As a whole, the values weakly increase along the

transects, although non-monotonically, from the marsh edge toward the inner

marsh, with some oscillations. An analysis of transect 1 (Fig. 2.3d) shows the

presence of two minimum values of organic matter density at 10 and 20 m away

from the channel (0.14 and 0.15 g cm-3, respectively), which seem to divide the

transect into three sub-intervals (from 0 to 10 m; from 10 to 20 m, and from 20 m to

the end of the transect).The maximum density values occur at 15 and 25 m from

the marsh edge, with values of 0.22 g cm-3. In the case of transect 2 (Fig. 2.4d) the

peaks in organic matter density are located at 2.5, 25, and 38 m from the channel

(with organic matter density values of 0.24, 0.26, 0.32 g cm-3, respectively). The


40

trend is characterized by two intervals delimited by minimum density values, in

which, with different slopes, the organic matter density increases, reaches a peak

and decreases (intervals from 0 to 7.5 m; from 7.5 m to 35 m). In the case of

transect 3 (Fig. 2.5d) two peaks of 0.19 g cm-3 occur at 15 and 30 m away from the

San Felice channel. The minimum values of organic matter density at 0, 25, 35 m

from the marsh edge, split the line in two intervals (from 0 to 25 m; from 25 to 35

m).

The results obtained with the H2O2 + NaClO treatment for each transect

show, in general, similar trends compared to the case of the H2O2 treatment (Figs.

2.3d, 2.4d, 2.5d). However, values obtained for the organic density are even

higher, the average value for the three transects being equal to 0.28 g cm-3. As to

the locations of the minimum and maximum values of organic matter density,

these are the same as those obtained with the H2O2 treatment and, along each

transect, the peaks emphasize the higher accumulation of organic matter in the

samples collected in correspondence of the low-lying pools (i.e. areas of minimum

surface elevations Fig. 2.1). The difference between the two methods tends to be

much larger in the samples that contain more organic matter.

The percentage amounts of organic matter, obtained by the different

methods at each given sampling site and calculated by the ratio between the mass

of the organic matter and the total mass for each sample (Fig. 2.6), further explain

the trends displayed by the organic matter for the different transects. The average

values for each method show well-defined and similar trends, regardless of the

method used. The organic matter percentage is minimum along the marsh edge

(6.2%, 10.8% and 18% for LOI, H2O2 and H2O2 + NaClO, respectively) and

increases rapidly to 17.3%, 25.4% and 36.7% for LOI, H2O2 and H2O2 + NaClO,

respectively, at about 5 – 7.5 m from the channel. The organic matter percentage

then tends to fluctuate up and down with no evident trend, the average values

from 5 m to the end of the transects being equal to 14.4%, 24% and 36% for LOI,

H2O2 and H2O2 + NaClO, respectively.


41

Fig. 2.6. Variability in the amount of soil organic matter, expressed as a percentage, with

distance from the San Felice channel. The empty markers represent the soil organic matter

values obtained by the three different methods, for each point along the transects. The full-

colored markers show the average of soil organic matter values for each point, for each method.

2.2.5.4 Above-ground biomass

The density of the vegetation cover (Figs. 2.3e, 2.4e, 2.5e) is maximum at 2.5

– 5 m away from the main channel (about 90%). From 10 to 30 m far from the

channel the density of the above-ground biomass shows lower and variable

values, with minimum values around 25 – 30 m from the marsh edges (values

range from 35% to 50%). Toward the end of the transects, in the last 10 m located

in the inner portion of the marshes, values of the above-ground biomass increase

noticeably for the two transects of the San Felice salt marsh, whereas the increase

is limited in the case of the Rigà transect. As a whole, the percentage amount of

vegetation cover may decreases more than 50% from the marsh edge toward the

inner marsh.


42

2.2.5.5 Soil Organic Carbon

In each transect the soil organic carbon density follows a different trend

(Fig. 2.7) but, as a whole, in the first 5 m from the San Felice channel it is lower

than in the inner part of the marshes. In the case of transect 1 the values fluctuate

up and down with minimum values of 0.037 g cm-3 on the marsh edge and 20 m

away from the channel, and a maximum value of 0.048 g cm-3 at 15 and 25 m from

the channel. In the case of transect 2 the increasing trend toward the inner marsh is

more defined and the values are spread over a wider range. The soil organic

carbon density is minimum on the marsh edge (0.027 g cm-3) and reaches a peak 30

m from the channel (0.063 g cm-3). In the case of transect 3 (minimum value of

0.028 g cm-3 at 5 m), the values increase up to a distance of 20 m from the edge

(peak of 0.054 g cm-3) and decrease over the last 20 m. The average value of soil

carbon density along the three transects is 0.044 g cm-3.

Fig. 2.7. Soil organic carbon density trend, for each transect, as a function of the distance from

the marsh edges toward the inner marsh.


43

2.2.6 Discussion

The marsh elevations along the three transects considered show that the

San Felice and Rigà salt marshes display a weakly concave-up profile, as

commonly observed elsewhere (e.g., Allen, 2000; Bartholdy, 2012; Sheehan &

Ellison, 2014). Field observations and model results show that sedimentation

patterns are characterized by a decrease in inorganic deposition with increasing

distance from the channels (French et al., 1995; Reed et al., 1999; Christiansen et al.,

2000; Temmerman et al., 2004; D’Alpaos et al., 2007). This behavior emerges as a

consequence of the progressive sediment settling as water moves towards the

inner marsh areas during the tidal flooding and of the concurrent decrease in

transport capability of tidal flows over the marsh as water approaches the no-flux

boundaries.

The sediment dry bulk density along the transects co-varies tightly with the

inorganic sediment content, which, in turn, is controlled by tidal advection of

suspended sediment. The inorganic sediment suspended by wind waves in the

shallow tidal flats (e.g., Carniello et al., 2011) is transported and deposited in the

proximity of the channel banks. The dense vegetation cover encountered by tidal

flood flows overspilling from the tidal channels, rapidly decreases flood current

velocities (e.g., Leonard and Luther, 1995; Yang, 1998) and turbulent kinetic

energy (Leonard and Croft, 2006; Mudd et al., 2010), thus promoting the

sedimentation of coarser suspended fraction in proximity of the channel edges

(Christiansen et al., 2000; Yang, 1998). Only finer sediment particles can therefore

be transported towards the inner portion of the marsh, as attested by the

progressive decrease in grain size along the transects in Figs. 2.3c, 2.4c, 2.5c. The

increasing elevations at the end of each of the transects are due to the presence of

two secondary tidal creeks, which, cutting through the inner parts of the marshes,

transport relatively fine-grained inorganic material that is deposited along their

banks. The formation of levees along these minor creeks shows that fine sediments


44

are transported through the smaller and inner reaches of the channel network and

are deposited on the marsh when water floods the platform.

The deposition of inorganic sediment plays an important role especially

along the marsh edges, where it dominates over organic accretion. If the marsh is

in equilibrium conditions, as suggested (see equations [1] and [2]), the total

accretion rate (𝑄𝑖 + 𝑄𝑜) needs to locally balance the rate of RSLR (that can be

assumed constant at the marsh spatial scale). Larger rates of inorganic deposition

should therefore be associated with lower rates of organic accretion. We note that

the decrease in the inorganic deposition with distance from the channel, already

suggested by both field observations and modeling results (French et al., 1995;

Reed et al., 1999; Christiansen et al., 2000; Temmerman et al., 2004; D’Alpaos et al.,

2007), is not always associated with a corresponding clear increase in the organic

content. LOI results show an almost constant organic matter content along the

transects, with the exception for the first meters along the channel levees

Interestingly, H2O2 and H2O2 + NaClO treatments, generally give a non-monotonic

increasing trend in the soil organic matter density and yield larger values of the

soil organic content than LOI. Moreover, they show that the maximum organic

density values occur at 15 and 25 m from the marsh edge. The fraction of organic

matter is minimum along the marsh edge, but it increases quite rapidly away from

the channel. In the inner part of the marsh, starting at about 5 – 7.5 m from the

channel levees, the ratio between the mass of the organic component and the total

mass, although much higher than on the channel levee, becomes almost constant,

suggesting that the organic component quickly becomes an important soil

component away from the main channel. The organic matter deposition is

controlled by plant productivity, but the organic matter accumulation in the soil is

not controlled just by the vegetation. The highest values in above-ground biomass

are observed on the channel levees (2.5 – 5 m away from the marsh edge), where

the elevation is higher and the organic content in the sub-surface samples is lower.

On the contrary, where the surface elevation is minimum, the density of the


45

vegetation cover is lower (a 50% decrease is observed – see Figs. 2.3e, 2.4e, 2.5e),

but the organic matter density in the samples is higher than near the edge. We

interpret this apparent contrast by noting that where the elevation of the marsh is

lower the soil is flooded for longer periods, possibly leading to increased soil

saturation and to hypoxic conditions. Without oxygen, the degradation of the

organic matter is much slower, such that a larger refractory fraction of the original

biomass persists, thus compensating for the reduced plant productivity. On the

contrary, the marsh edge, which is characterized by higher elevations, is flooded

for shorter periods and mainly during high tides. More prolonged drainage

conditions lead to more oxidized soils and to a faster organic matter degradation,

leading, in turn, to a lower organic accumulation in the soil.

In addition, it is worth emphasizing that the evaluation of the soil organic

matter content based on different methods highlights their different advantages

and the potential. The LOI method at temperatures between 350 and 375 °C is

likely to underestimate the amount of organic matter, in agreement with

Frangipane et al. (2009), because of its incomplete ashing (Donkin, 1991). The

treatment with NaClO following H2O2 turns out to be more effective in removing

the organic matter than H2O2 alone, although the effects of these two reagents on

the mineral fraction is still unknown.

Salt marshes, together with other coastal ecosystems like mangroves and

seagrass beds, are able to sequester CO2 from the atmosphere stocking high

quantities of carbon in the biomass and in sediments (Duarte et al., 2005; Mcleod et

al., 2011; Murray et al., 2011; Sifleet et al., 2011). This is because organic carbon in

the hypoxic marsh soils conditions is buried and preserved for a long period of

time, significantly contributing to the sequestration of “blue carbon” (Nellemann

et al., 2009; Mcleod et al., 2011). Blue carbon is sequestered over the short

(decennial) time scales in biomass, and over longer (millennial) time scales in

marsh sediments (Duarte et al., 2005), as marshes accrete vertically. Although they

cover an area <2% of the ocean surface, salt marshes, mangroves and seagrass beds


46

contribute close to half of the carbon burial in the coastal and global ocean (Duarte

et al., 2005). Unlike terrestrial forests, where the organic carbon stores are

dominated by the living trees, in vegetated coastal ecosystems the organic carbon

is stored in their organic-rich soils (Chmura et al., 2003; Duarte et al., 2005; Mcleod

et al., 2011; Murray et al., 2011). For example, salt-marsh systems by themselves

lead to a carbon burial rate more than two orders of magnitude higher than the

one observed in the tropical forest (Mcleod et al., 2011). When these fundamental

coastal ecosystems are degraded or converted to other uses by natural and/or

anthropogenic modifications, the sediment carbon is destabilized or exposed to

oxygen and it is released to the atmosphere or in the water column in the form of

CO2 (Pendleton et al., 2012). The organic carbon storage on salt marshes,

depending on their accretion rates, is geographically variable. In the case of the

northern Venice lagoon, considering that the San Felice salt marsh seems to be in

equilibrium with the forcing rate of RSLR (Carbognin et al., 2004; Marani et al.,

2007), an accretion rate of about 0.3 cm yr-1 can be expected (Day et al., 1998a). As a

consequence, from our measurements (see equation [3] for the determination of

the organic carbon), the average carbon accumulation rate for the San Felice and

Rigà salt marshes is estimated to be approximately equal to 132 g C m-2 yr-1. This

value is comparable with the results obtained by Chmura et al. (2003) for salt

marshes in the Gulf of Mexico, NE and NW Atlantic ocean, NE Pacific ocean

(average soil organic carbon: 0.039 g cm-3; average organic carbon accumulation in

the soil: 210 ± 20 g C m-2 yr-1), by Delaune and Pezeshki (2003) for coastal marshes

in Louisiana (183 g C m-2 yr-1) and by Duarte et al. (2005) who found, from a

compilation of previous reports published by Woodwell et al. (1973) and Chmura

et al (2003), an average carbon burial at the global scale for salt marshes of 151 g C

m-2 yr-1. Considering the loss in salt-marsh extension of about 110 km2 in the

Venice Lagoon which occurred over the last 100 years, and considering only the

organic carbon stocked in the topmost 1 m soil layer of the eroded marshes, it is

possible to estimate a release of almost 5 Tg of carbon in about 100 years. Again


47

considering only the topmost 1 m soil layer, the Venice lagoon salt marshes still

retain today a large carbon stock, amounting to about 2 Tg of C.

It is worth recalling that our estimates of soil organic carbon stocks and

fluxes were based on the results of the LOI analysis, in order to make them

comparable with previous and current literature results. Because LOI is found to

likely underestimate organic matter content compared to chemical treatments, the

evaluations of organic carbon accumulation and stock presented may constitute an

underestimation of the actual ones.

2.2.7 Conclusions

We analyzed variations in organic and inorganic soil production processes

at the marsh scale, a critical issue to improve our understanding of the mutual

feedbacks among processes of physical and biological nature in salt-marsh

landscapes and of the responses of salt marshes to a changing climate.

Our results emphasize that surface elevations, inorganic and organic

sediment content, and grain size distribution along marsh transects are tightly

related. In particular, our results show that coarser sediments are found along

channel levees (with a median grain size D50 in the range 33÷38 µm), while the

inner portion of the marsh is reached only by finer sediments (with a median grain

size in the range 12÷24 µm). This suggests that the tidal network which cuts

through the tidal landscape largely controls inorganic sediment transport over the

platform and further supports previous field observations and modelling results

emphasizing the occurrence of concave-up marsh surfaces with higher elevations

along the channel banks and progressively decreasing elevations towards the

inner portion of the marsh.

We also find that the amount of soil organic matter and inorganic content in

a sample is very sensitive to the specific analysis method (LOI, H2O2, H2O2 +

NaClO): LOI at 375°C for 16 hours underestimates the amount of the organic


48

sediment, while NaClO treatment (following H2O2 treatment) is likely to

overestimate it. Interestingly, we find that the measured organic matter density

provided by the H2O2 + NaClO method more than doubles the density estimates

obtained through LOI. To our knowledge this is the first study in which the results

obtained by using the above recalled methods are compared. We deem this to bear

important implications, as consequences it shows that the quantification of marsh

soil organic content and, consequently, of carbon stocks and, accumulation rates is

heavily dependent on the specific analysis method adopted.

We provide evidence that the accumulation rates of the organic and

inorganic components are related to the position on the salt marsh and to the

distance from the channels. In particular, we find that salt-marsh accretion is

mainly driven by the inorganic component in proximity of the channels, whereas

the organic component becomes important in the inner part of the marsh. We

suggest that our results, and similar determinations of the spatial distribution of

soil organic content, will prove useful to quantitatively inform and test marsh

biomorphodynamic models. The spatial distribution of organic soil can, in fact,

better constrain model representations of organic soil production and

decomposition, currently based on few point observations. In this respect, our

results show that the accumulation of organic matter in the soil does not increase

with biomass production, generally higher along the channel banks and lower in

the inner part of the marsh. This observation is consistent with the organic matter

accumulation in the soil being governed by the interplay of plant biomass

productivity and decomposition. This interplay is, in turn, modulated by soil

aeration, favoring decomposition, which is highest near the marsh edge and

lowest in the low-lying inner zones, where the reduced decomposition rate

compensates the lower biomass productivity.

Finally, our analyses provide the first estimates of soil organic carbon

density from LOI measurement (estimated on average to be 0.044 g C cm-3) and of

the carbon accumulation rate for marshes in the Venice lagoon (132 g C m-2 yr-1 on


49

average in the San Felice and Rigà salt marshes). These results highlight the

importance of salt marshes as sites of high atmospheric CO2 sequestration, and

carbon stock.

The proposed methodologies to determine the amount of soil organic

matter and inorganic content can be applied to other salt-marsh systems and

provides an effective method to study the coupled effects of physical and

biological processes at the marsh scale. We therefore believe our findings to be of

general interest and that the underlying biogeomorphic interactions shaping salt

marshes in Venice, which served as an illustrative case, are also responsible for the

evolution of salt-marsh systems worldwide.

Acknowledgements

This work was supported by the CARIPARO Project titled “Reading

signatures of the past to predict the future: 1000 years of stratigraphic record as a

key for the future of the Venice Lagoon”, and by the “Fondazione Ing. Aldo Gini”,

that are gratefully acknowledged.

This manuscript benefits of the constructive comments from Andrea

Rinaldo (Editor), Brad Murray and two anonymous reviewers.

Latest Holocene history of the southern Venice Lagoon

51

CHAPTER 3

LATEST HOLOCENE DEPOSITIONAL HISTORY OF THE

SOUTHERN VENICE LAGOON

3.1 OVERVIEW

This chapter is a journal paper in preparation. Sedimentological and

morphological analyses, integrated with chronostratigraphical data, were carried

out on salt-marsh, tidal-flat, subtidal-platform environments in the southern

portion of the Venice Lagoon. The results allow us to provide a detailed

description of the depositional history of the lagoonal sedimentary succession and

a detailed age model for the area over the last two millennia. This is a new aspect

that had not previously been provided for the Venice Lagoon, despite the large

body of literature which analyzed the history and evolution of the Lagoon.

3.2 PAPER

MARCELLA RONER1, ANDREA D’ALPAOS1, MASSIMILIANO

GHINASSI1, MARIAELENA FEDI2, LUCIA LICCIOLI2, LUCA GIORGIO

BELLUCCI3, LARA BRIVIO1

1Dept. of Geosciences, University of Padova, via Gradenigo 6, 35131

Padova, Italy.

2INFN Section of Florence, Via Rossi 1, 50019 Sesto Fiorentino, Italy.

3ISMAR-CNR, Via Gobetti 101, 40129 Bologna, Italy.


52

3.2.1 Abstract

The Venice Lagoon represents an outstanding example of man-landscape

co-existence. Among the typical lagoonal features, salt marshes are governed by

the interaction between physical and biological processes. Because of their unique

position in the tidal frame, salt marshes represent a crucially important ecosystem

providing valuable services to the environment. In the Venice Lagoon salt marshes

are currently exposed to possibly irreversible transformations due to the effects of

climate changes and human interferences, as in other cases worldwide. Analyzing

signatures of landscape changes in the stratigraphic record is crucial to refine our

knowledge of tidal landform dynamics and it represents a first step towards the

development of a predictive morphodynamic model. The southern Venice

Lagoon, where remarkable changes in extent of salt marshes and tidal flats have

been documented by historical sources, is suited to analyze modifications in the

depositional environment and, consequently, in the sedimentary record. We

collected 25 cores along a NE-SW linear transect about 5 km long cutting through

salt marshes, tidal flats and subtidal platforms. High resolution sedimentological

analyses defined the spatial arrangement of the four different deposits along the

transect, whose cores were dated through radiocarbon, 210Pb and 137Cs

geochronological analyses.

The study succession testifies an evolution from a palustrine fresh-water

environment to a lagoonal environment over the last 2’000 years. The depositional

history started with accumulation of palustrine peat which progressively evolved

into a salt-marsh environment in the 14th century. Salt-marsh aggradation is

characterized by different rates of accretion through time and occurred in parallel

with the decrease in the salt-marsh extent and tidal-flat expansion. Indeed, where

salt-marsh deposits were locally flooded and impacted by wind waves, a lag

deposit developed. As a consequence of the progressive water deepening, organic

rich mud accumulated above the lag. The results, as well as providing the first


53

accretion model for the latest Holocene succession in the southern Venice Lagoon,

highlight that the disappearance of salt marshes in this area of the Lagoon has to

be ascribed to the lateral erosion of their margins, rather than to a progressive

drowning due to the decrease in marsh elevation referred to MSL.

3.2.2 Introduction

Lagoon and estuarine environments are characterized by extremely high

biodiversity and elevated rates of primary productivity, host typical ecosystems

and landforms and important socio-economic activities worldwide (e.g., Cronk

and Fennessy, 2001; Barbier et al., 2011). Referring to the vertical position within

the tidal frame, supratidal areas are permanently dry, being located above the

Maximum High Water Level (hereinafter MHWL), whereas subtidal areas are

always submerged, lying below the Minimum Low Water Level (MLWL).

Intertidal areas are instead subjected to tidal fluctuations, their elevation ranging

between MHWL and MLWL. From a morphological point of view, salt marshes,

tidal flats and subtidal platforms represent the three major unchanneled lagoon

sub-environments. Salt marshes are topographically the highest, with elevation

between Mean Sea Level (hereinafter MSL) and MHWL. They are populated by a

luxuriant halophytic vegetation and display a generally flat weakly concave-up

profile (Adam, 1990). Tidal flats have an elevation between MSL and MLWL, they

lack any halophytic vegetation and are fully exposed only during exceptionally

low tides. Subtidal platforms are located below the MLWL, and are therefore

perennially submerged (e.g., Allen, 2000) These three environments are tightly

intertwined functionally and their morphological evolution, in both space and

time, is strictly related. For example, the extent, shape and elevation of tidal-flat

and subtidal-platform profiles in front of salt-marsh platforms can affect the

characteristics of wind-wave fields impacting salt-marsh boundaries and

promoting their erosion (e.g., Marani et al., 2010; Mariotti and Fagherazzi, 2013;


54

Hu et al., 2015). On the other hand the extent of salt marsh areas in front of tidal

flats or subtidal platforms controls fetch dimensions, thus directly influencing the

intensity of the wind-induced wave field (Mariotti and Carr, 2014). In addition,

sediments eroded from the bottom of tidal flats can be transported over the marsh

platforms by waves and tides and settle therein, while sediments eroded from salt-

marsh boundaries can be redistributed over the adjacent tidal flats or subtidal

platforms (Carniello et al., 2009). In the tidal frame, salt marshes represent an

important ecosystem providing valuable services to the environment (Mitsch and

Gosselink, 2000; Chmura et al., 2003; Costanza et al., 2008; MacKenzie and Dionne,

2008; Perillo et al., 2009; Davy et al., 2009; Gedan et al., 2009, Morgan et al., 2009;

Silliman et al., 2009; Barbier et al., 2011; Gedan et al., 2011; Mcleod et al., 2011;

Temmerman et al., 2013; Möller et al., 2014). The combined effects of natural

changes, such as sea level rise, subsidence, erosion, and human interferences, such

as reduced sediment supply, expose these ecosystems to possibly irreversible

transformations. These effects turned out in an extensive loss of global

marshlands, especially during the last century (e.g., Day et al., 2000; Marani et al.,

2007; Gedan and Silliman, 2009; Gedan et al., 2009; Kirwan et al., 2010; Marani et

al, 2010; Mudd, 2011; Pendleton et al., 2012).

The same fate afflicts salt marshes in the Venice Lagoon, a process also

associated to a general expansion and deepening of tidal flats and subtidal

platforms (Marani et al., 2007; Carniello et al., 2009; D’Alpaos 2010a; D’Alpaos et

al., 2013). The current morphology of the Venice Lagoon is the result of a

Pleistocene – Holocene evolution affected by both changes in the environmental

forcings and, in the last millennium, by intense human modifications. The

Holocene history of the Venice Lagoon has been well documented in recent

studies and has arisen from the employment of very high-resolution seismic data

integrated with core analyses (Brancolini et al., 2008; Zecchin et al., 2008, 2009,

2011, 2014; Tosi et al. 2009a; Madricardo and Donnici, 2014). Although the

accumulation of the lagoonal succession started around 7’000 – 6’000 years B.P.,


55

only during the last 2’000 years the main changes in depositional patterns were

strongly influenced by the interaction between natural processes, such as local

subsidence and sea level rise (e.g., Day et al., 1999; Carbognin and Tosi, 2002; Tosi

et al., 2002; Brambati et al., 2003; Carbognin et al., 2004, 2005; Teatini et al., 2005,

2012) and human interventions, such as river diversions and channel dredging

(e.g., Gatto and Carbognin, 1981; Favero, 1985; Carbognin, 1992; Ravera, 2000;

Brambati et al., 2003; Tosi et al., 2009a, 2009b; D’Alpaos, 2010a, 2010b; Bondesan

and Furlanetto, 2012). During the last century, the exploitation of underground

water significantly contributed to dramatic changes in subsidence of the Venice

Lagoon (e.g., Carbognin, 1992; Brambati et al., 2003; Carbognin et al., 2005) and

further activities associated with land reclamation and fish-breeding caused a

significant decrease in the lagoon surface.

The main environmental and morphological changes occurred in the Venice

Lagoon over the past centuries are well known from historical sources, which are

represented by historical maps and archive documents (Dorigo, 1983; Favero et al.,

1988; Dorigo, 1994; D’Alpaos, 2010a, 2010b). Nevertheless, relatively poor

attention has been paid to detecting the signature of these changes in the lagoonal

sedimentary record, which commonly appears to be significantly expanded given

the remarkable accumulation rate (i.e., 0.25 cm yr-1) that characterizes the Venice

area (Bellucci et al., 2007). Lucchini et al. (2001) analyzed physical and chemical

features of sediments (such as grain size, mineralogy, major and trace elements,

organic carbon and total nitrogen) and the mechanism which control them within

the topmost 60 cm of sediment cores within salt-marshes, tidal flats, and subtidal

platforms in the Venice Lagoon They pointed out that sediment characteristics

from the Northern Venice Lagoon reflect the occurrence of steady conditions for

sediment supply and hydrodynamic and wave forcings, while sediment

characteristics from the central and southern Venice Lagoon reflect an opposite

trend. Bellucci et al. (2007) determined the depositional history of salt-marsh

deposits accumulated over the past century analyzing five cores recovered from


56

different sites in the lagoon. They suggested that, on the long term, the rate of

relative sea level rise is the driving forcing for marsh accretions, whereas, on the

short term, variations in wind patterns, storm frequency and climate are the

driving forcings for salt-marsh accretion rates in the Venice Lagoon.

The present paper focuses on the latest Holocene sedimentary succession of

the southern Venice Lagoon, where remarkable changes in extent of salt marshes

and tidal flats have been documented by historical sources (e.g., D’Alpaos, 2010a,

2010b). This work aims at detecting the signature of these changes in the

stratigraphic record through a multidisciplinary approach based on

sedimentological and geo-chronological analyses. Specifically, the aim of the

paper is twofold: i) to define the sedimentary features and stratigraphic

architecture of the lagoonal succession; ii) to provide a detailed age model for the

latest Holocene salt-marsh succession.

3.2.3 Geological setting

The Venice Lagoon (Fig. 3.1A) is located in a foreland region comprised

between the NE-verging northern Apennine and the SSE-verging eastern South-

Alpine chains. It is a shallow (average depth 1.5 m) lagoon with a total extent of

550 km2, and a semi-diurnal micro-tidal regime with maximum water excursion of

± 70 cm around Mean Sea Level (hereinafter MSL). The Venice Lagoon is

connected to the Adriatic sea through three inlets: from north to south, Lido,

Malamocco and Chioggia.

The Venice Lagoon formation started about 7’000 – 6’000 years B.P. in

correspondence of the maximum marine transgression triggered by the Holocenic

sea level rise (Fontana et al., 2004), which produced flooding of a late Pleistocene

alluvial system developed during the Last Glacial Maximum. During the marine

ingression, between 10’000 and 6’000 years B.P., longshore drift currents triggered

the formation of a barrier island that delimited the Venice paleo-Lagoon


57

(Brancolini et al., 2008). The Holocene sedimentary succession unconformably

overlays the late Pleistocene alluvial deposits and is composed of three main

seismic units separated by major stratal surfaces (Zecchin et al., 2008). The first

unit accumulated under transgressive conditions and resulted from the infill of

estuarine and/or fluvial channels. The second unit accumulated during highstand

conditions and is composed of a prograding wedge consisting of shoreface-shelf

deposits and ebb tidal deltas, which pass landward into back-barrier sediments.

The third unit also accumulated during the highstand phase and mainly consists

of lagoonal deposits associated with development of a complex tidal network.

Figure 3.1. (A) Location of the study area in the southern Venice Lagoon, Italy. (B) Location of

the 25 cores in the areas of Valle Millecampi, Punta Can salt marsh, Fondo dei Sette Morti.

The study area is located in the southern portion of the Venice Lagoon (Fig.

3.1A), where the Holocene transgression took place from ~10’000 to 6’000 years

B.P. (Zecchin et al., 2009) accumulating a 20-22 m thick sedimentary succession

(Tosi et al., 2007a, 2007b; Brancolini et al., 2008; Zecchin et al., 2008, 2009). During

the past millennium, this area received considerable amount of clastic sediments

from the Brenta River (e.g., Fontana et al., 2004; Tosi et al., 2007b; Amorosi et al.,

2008; Tosi et al., 2009a), which was repeatedly diverted from and re-introduced in

the lagoon (e.g., D’Alpaos, 2010a, 2010b; Bondesan and Furlanetto, 2012) during

the last centuries. The diversion of the main rivers out of the Venice Lagoon


58

caused remarkable changes in salt marsh extent: salt-marsh areas, in fact,

decreased from about 180 km2 in 1811 to about 50 km2 in 2002, with a reduction of

more than 70% (Marani et al., 2003, 2007; Carniello et al., 2009; D’Alpaos, 2010a)

3.2.4 Methods

A total number of 25 sedimentary cores (1.0 to 1.5 m long) was recovered

along a NE-SW trending, 5.2 km long transect (Fig. 3.1B). This transect crosses the

Punta Cane salt marsh and the Valle Millecampi and Fondo dei Sette Morti tidal

flat and subtidal platforms (Fig. 3.1B). Study cores were recovered from different

setting (Fig. 3.2A): 11 cores from salt marshes; 5 cores from tidal flats; 9 cores from

subtidal platforms. Cores are 10 cm in diameter and up to 1.5 m long, and were

obtained stabbing vertically a cylindrical steel corer into the sediment. Complete

recover of the core was allowed by a mechanical system, which shut off of the

bottom of the corer. Recovering of cores with a diameter of 10 cm was required in

order to collect a significant volume of sediment for geochronological analyses

(e.g. find enough charcoal/wood fragments for radiocarbon dating). Since vertical

stabbing of the corer caused sediment compaction, and additional core was

recovered for each site using an auger corer (3 cm diameter), which prevents

sediment compaction. Comparison between the two cores allowed us to define the

original thickness of different sediment layers in the main core. If not differently

specified, de-compacted thicknesses are used in the text. Elevation and geographic

location of each core were determined using two TOPCON GR–3 GPS receivers

(dual frequency - L1/L2 - and dual constellation - NavStar/Glonass - with

integrated Tx/Rx UHF radio). Each core was halved lengthwise in laboratory. The

first half was used for sedimentological and geochronological analyses, whereas

the second one was archived.


59

Fig

ure

3.2

. (A

) D

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th

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.


60

3.2.4.1 Sedimentological analysis

A high-resolution sedimentological analysis was carried out on the study

cores through the principles of modern facies analysis in order to define the

distinctive features of different types of deposits and to link them with

corresponding sedimentary processes and depositional environments. Different

types of deposits were differentiated basing on their color, grain size, texture,

sedimentary structures and macroscopical biogenic content, that mainly consisted

of shells, plant debris and in situ vegetal remains. An high-resolution stratigraphic

framework was finally obtained correlating different types of deposits and related

bounding surfaces along the transect.

3.2.4.2 Geochronological analyses

The Punta Cane and Fondo dei Sette Morti areas (Fig. 3.1B) were selected

for geochronological analyses, since density and depth of core data allow one to

investigate the maximum thickness of deposits with a remarkable resolution (Figs.

3.2A, 3.2B). Geochronological analyses were performed through radiocarbon and

radionuclides (137Cs and 210Pb) analyses, which were performed on three selected

cores (cores 1, 28, 11). These cores can be laterally correlated in order to define a

composite core with an overall thickness of about 3.5 m. The Valle Millecampi area

was selected for determining the radiocarbon dating of the onset of tidal

flat/subtidal platform deposits on samples arising from core 14.

3.2.4.2.1 Radiocarbon analyses

Samples for radiocarbon analyses were collected from cores recovered in

the Valle Millecampi (2 from core 14), in the Punta Cane (3 from core 1; 3 from

core 28) and in the Fondo dei Sette Morti (1 sample from core 11) areas. Samples

from Valle Millecampi were picked up to establish the onset of tidal flat/subtidal


61

platform deposition, whereas those from Punta Cane and Fondo dei Sette Morti

were collected through the whole study succession in order to frame in time the

main changes in depositional dynamics and sediment aggradation rate. Samples,

consisting either of charcoals or vegetal remains (e.g., fragments of reeds, wood

pieces, fragments of stems from halophytic plants in life position) were measured

by 14C – AMS (Accelerator Mass Spectrometry). The datable materials were

recovered by directly picking from the cores for the coarser material and by

floating for the finer fragments.

Before the AMS measurement, samples were cleaned using the so-called

Acid-Base-Acid (ABA) procedure that allows one to remove any natural

contaminations possibly due to carbonates and humic traces. Then carbon was

first extracted as gaseous CO2 by combustion and finally converted to solid

graphite by reaction with hydrogen, in the presence of iron powder as catalyst.

According to the typical laboratory quality check procedure, two graphite pellets

were prepared from each of the pre-treated samples. AMS measurements were

performed exploiting the dedicated beam line at the 3 MV Tandem accelerator

installed at INFN-Labec, Florence (Fedi et al., 2007). 14C/12C isotopic ratios were

measured in unknown, standard and blank samples, as well as 13C/12C ratios (to

correct for isotopic fractionation). After verifying their consistency from the

statistically point of view, for each sample, the best estimate of the radiocarbon

concentration, and as a consequence of the radiocarbon age, was calculated as the

weighted average of the two measured pellets.

3.2.4.2.2 Radionuclides 210Pb and 137Cs

Geochronological analyses through 210Pb and 137Cs were carried out on the

uppermost 70 cm sedimentary interval of core 1. This thickness is referred to the

compacted core, and corresponds to 85 cm of de-compacted salt-marsh deposits.


62

Alpha spectrometry of 210Po was used for 210Pb determinations, assuming

secular equilibrium between the two isotopes. 210Po was extracted from the

sediment with hot HNO3 and H2O2, in the presence of 209Po as a yield monitor, to

account for extraction and counting efficiencies. After separation of the leachate

from the residue, the solution was evaporated to near dryness and the nitric acid

was eliminated using concentrated HCl. The residue was dissolved in 1.5 N HCl,

and Po was plated onto a silver disk overnight, at room temperature. Iron was

reduced using ascorbic acid (Frignani and Langone, 1991), and alpha decays were

counted by a silicon surficial barrier detector connected to a multichannel

analyser. The analyses of the same sample with the two different 209Po internal

standards used at ISMAR (Bologna) and MSRC (Stony Brook) gave nearly

identical results, thus suggesting that the analyses were accurate. Nevertheless,

the accuracy of 210Pb analyses was estimated also by repeated measurements of the

certified standard sediment IAEA 300 (Baltic Sea sediment) and the results were

within the standard uncertainties. In addition a successful intercalibration in the

framework of the Euromarge-NB project was carried out (Sanchez-Cabeza et al.,

1994). Precision, calculated from independent analyses of the same sample, was

4.6%.

137Cs was measured by non-destructive gamma spectrometry of dry

samples in standard vessels of suitable geometries. The analytical accuracy was

periodically checked by counting the international certificate standard IAEA Baltic

Sea sediment, and the results were within the standard uncertainties. In addition,

two international intercalibrations (IAEA Proficiency test: Determination of

Anthropogenic g-emitting Radionuclides in a Mineral Matrix, 2002; IAEA

Proficiency test: Determination of γ-emitting Radionuclides, 2006) yielded 137Cs

activities within 4.1% and 1.6% of the IAEA accepted values, respectively.

Precision, estimated by repeated analyses of the same sample, ranged between

2.05 and 3.07%. Efficiencies on 10 ml geometries (3.12-3.18%) were calculated

through a series of standards obtained by spiking old sediment with a known


63

amount of the Amersham QCY58 multi-peak standard solution. The analytical

detection limit for 137Cs was 3 Bq kg-1.

3.2.4.3 Accretion model

Calibration of the measured radiocarbon ages was performed exploiting the

Bayesian inference. For the samples of the Punta Cane and Fondo dei Sette Morti

areas, a chronological model was a priori built using both the information about

the chronological order of the events and the sampling depth of the dated

materials. The OxCal 4.2 (Bronck Ramsey, 2008) software was used. Because we

can expect that the deposition process was random, the P_Sequence model was

applied. In this model, we assume that, along the dated succession, the deposition

rate may have random fluctuations (according to Poisson statistics). We can

choose the guessed number of the accumulation events per unit depth (the so-

called k parameter in OxCal): the higher is k, the more uniform is the expected

sedimentation rate. In this case, we assumed a k parameter of 0.1 cm-1

(corresponding to 1 accumulation event per 10 cm depth). In defining our model,

we also allowed the model to estimate the age distributions of probability of

events that had not to be directly dated, considering them every 20 cm. The

IntCal13 calibration curve (Reimer et al., 2009) was used in OxCal as reference.

Finally, our likelihood was build integrating the radiocarbon data with

samples dated by 137Cs and 210Pb series.

3.2.5 Results

The study transect is characterized by an overall topographic relief of about

2.5 m below MSL (Fig. 3.2A). Salt marsh areas show an elevation ranging between

16 and 41 cm above MSL, with the lower values located in the Punta Cane area.

Salt marshes are surrounded by bare tidal flats which extend up to 70 cm below


64

the MSL (Fig. 3.2A). A maximum water depth of 1.3 and 2.16 m below the MSL

was measured in the Valle Millecampi and Fondo dei Sette Morti, respectively,

where constantly flooded subtidal platforms develop (Fig. 3.2A).

3.2.5.1 The study deposits

3.2.5.1.1 Sedimentology

Five types of deposits were recognized through sedimentological analysis .

The deposits are described and interpreted in the following.

i. Palustrine deposits

They are at least 2 m thick and consist of peat with abundant fragment of

reeds, which can be up to 15 cm long (Fig. 3.3A). They occur in all the cores

exceeding about -1.5 m below MSL (Fig. 3.2B). Peat consists of comminuted plant

debris, is massive and contains a minimum amount of dispersed mud and very

fine sand. Rare reeds in sub-vertical positions have been recovered from core 10.

The peat can either grade upward into salt-marsh mud or be abruptly overlain by

a shell-rich lag. The basal part of the peat was not encountered in any core.

In the NE sector of the Valle Millecampi subtidal platform, two adjacent

cores show that sandy deposits occur laterally to the peat (cores 13a and 13 b in

Fig. 3.2B). Only a 15 cm thick layer of sandy deposits has been recovered and

consists of medium sand grading upward into a very fine sand (Fig. 3.3C).

Medium sand is well-sorted and contains scarce matrix, whereas fine sand is rich

in mud and contains abundant plant debris.

Peat deposits are interpreted to be formed in a palustrine setting, where

vegetal remains were transported as debris or produced in situ, as attested by the

occurrence of sub-vertical reeds, which probably were buried in life position. In

this framework, the sandy deposits occurring in the NE sector of the Valle

Millecampi area could represent the infill of a fluvial/distributary channel


65

draining a densely vegetated coastal plain. The occurrence of a progressive fining-

upward of the sandy unit, suggesting the progressive deactivation of the channel,

supports this hypothesis.

ii. Lag deposits

These deposits are up to 15 cm thick and occur only in cores located in

subtidal platforms and tidal flat areas. They consist of fine sand fining upward

into muddy silt. Sand is very rich in shells and shell fragments (Fig. 3.3B). Bivalves

are commonly disarticulated, although few specimens of Cerastoderma edule have

been found in life position. Sand is mainly massive, although a subtle horizontal

layering can be locally distinguished. Wood fragments, up to 1 cm in size, are also

common. Bioturbation is intense, although distinct burrows can be rarely

distinguished. These shell-rich sand overlay palustrine peat and grade upward

into muddy subtidal platform deposits. They can be followed for several hundred

meters along the study transect (Fig. 3.2B).

The absence of mud and concentration of shells are consistent with lag

deposits produced by wave winnowing. The occurrence of fragmented and

disarticulated shells, along with the considerable lateral extent, the fining-upward

grain size trend and the abrupt basal surface, strongly support this hypothesis.

iii. Tidal-flat/subtidal-platform deposits

They are up to 1.30 m thick and consist of dark, organic-rich mud with

subordinate sandy layers (Figs. 3.3D, 3.3E). Mud is massive and contains scattered

plant fragments. Shells can be isolated or form localized concentrations (Fig. 3.3D).

Bivalve Cerastoderma edule frequently occurs in life position. Sandy layers are up to

2-3 cm thick and commonly show a lenticular geometry with a flat base and wavy

top (Fig. 3.3E). They consist of fine to very fine moderately to well-sorted sand and

are frequently disturbed by intense bioturbation. These deposits occur only in

cores located in subtidal platforms and tidal flat areas and conformably overlie

shell-rich lag sand.


66


67

Figure 3.3. Sedimentary facies of the study deposits. (A) Peat deposit with fragment of reeds

from millimetric to decimetric in size. (B) Shell-rich lag deposit. (C) Sandy to muddy-sandy

paleo-channel deposit. (D) Massive tidal-flat/subtidal-platform deposit with shells. (E)

Laminated tidal-flat/subtidal-platform deposit. (F) Sub-millimetric fine-medium sand layers in

salt-marsh brownish silt deposit. (G) Organic material organized in layers in salt-marsh

brownish silt deposit. (H-I) Plant debris in life position in the salt-marsh brownish deposit.

Dominance of mud indicates settling from sediment suspension in a low-

energy setting, which is consistent with a tidal-flat or subtidal-platform

depositional environment. Sandy layers and localized shell concentration

developed during storm events, when wave-generated currents suspended the

mud causing concentration of the coarser deposits on the depositional interface.

The wavy top of sandy layers indicates local preservation of wave-generated

ripple forms and supports the previous interpretation.

iv. Salt-marsh deposits

These deposits are up to 2.0 m thick and occur only in cores recovered in

salt-marsh areas. They consist of a horizontally-laminated, bioturbated, brownish

mud, with a variable amount of fine to very fine sand (Figs. 3.3F, 3.3G). Several

mud laminae are dark brown and appear to be very rich in plant debris (Fig. 3.3G)

and wood fragments. Sand is mainly concentrated in millimetric, whitish,

horizontal laminae (Fig. 3.3F), which are characterized by a good grain-size

sorting. Roots and vertical stems up to 0.5 cm in diameter are common (Figs. 3.3H,

3.3I).

These deposits accumulated in salt-marsh environment, in the highest

portion of the intertidal zone that was commonly affected by subaerial exposure

allowing the growth of a dense halophytic vegetation. Muddy deposits likely

settled down around high water slack, at the transition between flood and ebb

tide. Sandy laminae were generated during storm events in high tide conditions,

when wind-generated waves winnow the salt-marsh surface, suspending mud


68

and concentrating sand. Textural sorting of sandy laminae and lack of a muddy

matrix support this hypothesis.

3.2.5.1.2 Stratigraphy

The substrate of the study succession is made of palustrine peat, that is

documented almost along the whole transect (Fig. 3.2B). Above the basal peat both

salt-marsh and tidal-flat deposits are documented. In the Valle Millecampi area,

the peat is abruptly overlaid by the shell-rich lag deposit (see, for example, core 26

in Fig. 3.4), which shows maximum thickness in the central, deepest parts of the

depression and becomes thinner toward its margins (Fig. 3.2B). The lower

boundary of the shell-rich lag is commonly disrupted because of intense

bioturbation (e.g., core 26 in Fig. 3.4). Muddy tidal flat/subtidal platform deposits

Fig. 3.4 Sedimentological log of core 28: this core exhibits the transition between the basal peat

and the salt-marsh deposit, which is characterized by a gradual decrease in the reeds content

and an equivalent increase in the mud and halophytic roots content. Sedimentological log of

core 26, typical tidal-flat and subtidal-platform core of Valle Millecampi. The presence of

widespread bioturbation marks the passage from the basal peat to the shell-rich lag, the latter

representing the base of the organic-rich, blackish, massive or laminated mud.


69

are widespread in the Valle Millecampi area above the shell-rich lag. These

deposits are characterized by an overall fining-upward grain size trend from fine

sand to mud (e.g., core 26 in Fig. 3.4). Where salt marshes are well developed, the

basal peat grades upward into salt-marsh deposits. The transition occurs within

about 20 cm and is characterized by a gradual decrease in content of reed

fragments, which occurs in parallel with an increase in root traces and mud

content (i.e., core 28 in Fig. 3.4). In the Fondo dei Sette Morti area, the shell-rich lag

and overlying mud are missing, and the basal peat and salt marsh deposits are

exposed at the depositional interface (Fig. 3.2B).

3.2.5.2 Radiocarbon datings

Table 3.1 shows the experimental results of 14C-AMS measurements

performed in Valle Millecampi, Punta Cane and Fondo dei sette Morti areas. For

each sample, the best estimate of the radiocarbon concentration, and the

corresponding radiocarbon age, were calculated as the weighted average of the

two measured graphite pellets (see the Lab. codes column in Table 3.1). Only in

the case of samples 14.30 and 1.112, the low amount of the residual mass after the

ABA pre-treatment allowed us to prepare just a single graphite pellet. Data are

quoted at 1 sigma uncertainty.

Samples from Valle Millecampi, that were collected from the shell-rich lag

to establish the onset of the deposition in this basin, provided low-quality results.

This is probably due to the reworking of charcoal and wood materials during the

formation of the wave-winnowed lag. As a whole, despite the significant errors in

the calibrated ages (sample 14.33: 240−105+85 cal AD; sample 14.30: 265−125

+155 cal AD,

ages expressed as mode value in the interval at 68% of probability), these results

would suggest that the age of lag deposits would span between the 2nd and 5th

century.


70

Recovering

area Sample

Lab.

codes

Depth

(cm referred

to MSL)

14C conc.

(pMC)

Radiocarbon

age

(years BP)

Valle

Millecampi

14.33 14Fi2560

14Fi2566 -250.0 80.05 ± 0.57 1790 ± 55

14.30 14Fi2569 -241.0 80.6 ± 1.1 1730 ± 110

Punta Cane

28.33 14Fi2627

15Fi2632 -208.0 90.32 ± 0.47 820 ± 40

28.19 14Fi2561

14Fi2565 -165.2 92.64 ± 0.54 615 ± 45

28.11 14Fi2638

14Fi2640 -140.7 94.25 ± 0.38 475 ± 30

1.112 14Fi2553 -113.3 94.50 ± 0.68 455 ± 60

1.80 14Fi2625

15Fi2628 -75.5 96.49 ± 0.36 287 ± 30

1.70 14Fi2624

14Fi2629 -63.7 96.82 ± 0.49 260 ± 40

Fondo dei

Sette Morti 11bottom

14Fi2559

14Fi2564 -342.0 77.70 ± 0.50 2030 ± 50

Table 3.1. Experimental data of 14C-AMS measurements.

In the Punta Cane and Fondo dei sette Morti areas, the seven dated samples

are distributed along the composite core 1-28-11 at different depths (Fig. 3.5),

which are referred to MSL. The age for each sample is represented by the mode

value of the calibrated age quoted at 68%. As a whole, radiocarbon datings show

that the oldest peat deposits occur at about 3.40 m below MSL, and that they are

dated at about 0±60 cal AD. The transition between palustrine and salt-marsh

deposits is dated at 1355−45+40 cal AD.


71

Figure 3.5. Detailed representation of the seaward part of the study transect. The sedimentary

succession involves the peat deposit which gradually passes to the salt-marsh deposit. The

yellow dots represent the uncompacted depths of the dated samples. Each age reports the

employed geochronological method (14C, 210Pb, 137Cs), the mode value obtained from the age

distribution, and the errors of the calibrated ages interval at 68% of probability (see Table 3.2).

3.2.5.3 Radionuclides 210Pb and 137Cs

The 210Pb activity-profile of core 1 (Fig. 3.6A) reaches, on the compacted

core, the equilibrium depth at 55 cm from the core top and allows us to set the

1900 ± 10 AD at 50 cm, that corresponds to 60 cm from the core top of the de-

compacted one. The 137Cs activities show a clear trend (Fig. 3.6B) and the peaks at

7.5 cm and at 25.5 cm (9 cm and 30.5 cm on the de-compacted core, respectively)

are associated to 1986 AD (fallout from the Chernobyl accident) and to 1963 AD

(maximum 137Cs fallout from nuclear testing), respectively.

3.2.5.4 Accretion model

As already recalled above, calibration of the measured radiocarbon ages

was performed using the P_Sequence model and adding to the dated succession

the more recent samples dated by Pb and Cs series: Pb 1900±10 AD, Cs1 1963 AD,

Cs2 1986 AD. Running the model gives us satisfying results: the agreement index

is indeed 99%, with the individual agreement index of each sample well above the


72

Figure 3.6. 210Pb and 137Cs activity-depth profiles in core 1.

threshold of 60%. Table 3.2 shows the calculated intervals for the calibrated age of

the measured samples, quoted at both 68% and 95% probability levels. The depths

of the samples, referred to MSL, were calculated on the basis of the de-compaction

process after calibration of the depth of the different deposits.

Figure 3.7 shows the data evaluated applying the P_Sequence model using

the mode of the calculated distribution of probability for each sample. In fact, even

though it is well known that there is no good point estimation of the true calendar

age of a sample dated by radiocarbon, the mode of the distribution may be taken

as an acceptable approximation (MichczyÒski, 2007). We can therefore use this

information to evaluate the sedimentation rate. The figure clearly shows that the

accumulation rate is not constant over the last two millennia.


73

Sample Depth

(cm referred to MSL)

Cal Age (BC/AD)

68% prob.

Cal Age (BC/AD)

95% prob.

11bottom -342.0 -60 – 60 -165 – 80

28.33 -208.0 1160 – 1240

1045 – 1085 (8%)

1125 – 1135 (1%)

1150 – 1270 (86%)

28.19 -165.2 1310 – 1395 1290 – 1405

28.11 -140.7 1420 – 1445 1405 – 1450

1.112 -113.3 1445 – 1515 1430 – 1585

1.80 -75.5 1580 – 1595 (3%)

1615 – 1660 (65%) 1525 – 1665

1.70 -63.7 1635 – 1670 (61%)

1785 – 1795 (7%)

1545 – 1595 (4%)

1615 – 1685 (74%)

1735 – 1805 (17%)

Pb -41.1 1886 – 1908 1877 – 1917

Cs1 -11.6 1962 – 1964 1960 – 1965

Cs2 10.0 1984 – 1988 1982 – 1990

Table 3.2. Calibrated ages of the measured samples, quoted at both 68% and 95% probability

levels, as calculated applying the P_Sequence model. When more than one interval was

calculated for each level of confidence, the probability for each definite interval is also shown in

brackets.

The aggradation trend, derived from the age model shown in Figure 3.8A,

is portrayed in Figure 3.8B, in which the total accretion rate is plotted as a function

of the depth along the succession (the inset shows the accretion rate as a function

of time). The mode and the errors employed are represented by the result of the

calibrated ages intervals at 68% of probability.

As a whole, the sedimentation rate along the study succession considering

the mode values tends to gradually increase from the bottom to 1480 AD, with

increasing values starting from 0.11, 0.23, 0.33, up to 0.55 cm/yr. During the 16th

century, framed from a depth of 113 cm to a depth of 75 cm below MSL along the


74

Fig. 3.7. Calibrated ages of the measured samples plotted versus the sampling depth. For each

sample the mode of the posterior distribution of probability as determined applying the

P_Sequence model is shown; error bars indicate the extremes of the calibrated ages interval at

both 68% (in red) and 95% (in blue) of probability.

succession, the accumulation rate decreased to 0.24 cm/yr. The above interval is

characterized by an important increase in the accumulation rate which reaches

values of 1.18 cm/yr, and it is later followed by a significant decrease. During the

ages from 1650 AD to 1986 AD the sedimentation rate increases again, changing

from values of 0.09 cm/yr until the end of the 19th century, to values of 0.43 cm/yr

from 1900 AD to 1963 AD, up to 0.94 cm/yr in the years between 1963 AD and

1986 AD. From the year of Chernobyl accident to nowadays the accretion rate on

Punta Cane salt marsh is equal to 0.33 cm/yr, which is much smaller compared to

the value of 2.32 cm/yr obtained by Day et al. (1998b) for the same study area.


75

Figure 3.8. Accumulation rate model. On the left (A) the depths of each dated sample are

represented in a schematic continuous sedimentary succession built up from cores 1, 28 and 11,

after the de-compaction process. Each sample age is expressed as the mode value in the

distribution and the two errors represent the ages interval at 68% of probability. On the right (B)

the accumulation rates are plotted versus the depth intervals. The black line represents the

accumulation trend considering the mode values; the two dashed lines represent the minimum

and the maximum accumulation rates considering the result of the calibrated ages intervals at

68% of probability. The inset shows the accretion rate as a function of time.

3.2.6. Discussion

3.2.6.1 Depositional history of the Punta Cane area

The radiocarbon age of the deepest sample indicates that, during the 1st

century BC, the NE sector of the study transect (i.e., the Punta Cane area) was

characterized by peat accumulation in a palustrine environment. Palustrine

sedimentation persisted for about 1,400 years allowing the accumulation of about

2.0 m of peat. Continuous peat sedimentation indicates that organic deposition


76

faced the increase in accommodation space also during the 12th and 13th century,

when combination between sea-level rise and local subsidence caused an increase

in mean sea level. Sandy deposits which were observed in the SW part of Punta

Cane area (see cores 13a and 13b in Fig. 3.2B) suggest that clastic sediment

probably remained confined within distributary channels (e.g., Horne et al., 1978).

The presence of peat and of small sandy deposits, agrees with information derived

from historical sources pointing out that the Brenta River was active (although

being characterized by the presence of different reaches and different outlets

within the Lagoon), during the 1st century BC, during the Roman age, and between

the 5th and the 9th century AD (Mozzi et al., 2004). Moreover, across the studied

transect (Fig. 3.1B), the presence of a generally peaty continental substratum of

Roman age and the presence of organic and salt-marsh deposits of Late-Roman –

Medieval age, lying above this substratum (Tosi et al., 2007b), confirms the

presence of an active reach of the Brenta River, although it is difficult to establish

which Brenta reach they belong to.

During the 14th century, the palustrine environment gradually experienced

a transition into a salt marsh system. Transitions of this type might be the result of

a decrease in the rate of relative sea level rise, of an increase in sediment supply, or

both (Marani et al., 2010; Kirwan et al., 2011; Mudd, 2011). We are not aware of

relative sea level reconstructions dating back to this period, nor of diversions of

the Brenta River likely promoting an increase in the availability of sediment. In

addition, in the case of the Venice Lagoon, to our knowledge, no historical maps of

the Lagoon exist dating back to before 1556 AD (Cristoforo Sabbadino Map, Fig.

3.9A). Interestingly, our results document, for the first time, the transition from a

palustrine to a salt-marsh system in this portion of the Venice Lagoon. Salt marsh

deposition persisted until the present day allowing the accumulation of about 1.80

m of sediment in about 650 years, although the accretion rates over time are not

constant.


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3.2.6.2 Depositional history of the area landward of Punta Cane

As to the Valle Millecampi area (see Figs. 3.2A, 3.2B), located landward of

the Punta Cane area, the basal peat is overlaid by a lag deposit over which a mud

deposit further accumulated. The age of the wave-winnowed lag deposits suggests

that flooding of the Valle Millecampi area occurred between the 2nd and 5th century

AD. As noted above, the first historical map showing the presence of a tidal flats

in this area dates back to the 1556 (Cristoforo Sabbadino map, Fig. 3.9A). During

the 16th century, the Valle Millecampi basin was already well-developed, thus

suggesting that the fetch and water depth were large enough for the development

of waves capable to produce bottom shear stresses promoting resuspension of

sediments from the bottom (Fagherazzi et al., 2006; Francalanci et al., 2013;

Mariotti and Fagherazzi, 2013). Although radiocarbon calibrated ages are affected

by a remarkable error, they are consistent with observations derived solely based

on historical maps. (e.g., D’Alpaos 2010a, 2010b). The overall fining-upward grain

size trend of the deposits overlying the lag points (see core 26 in Fig. 3.4) are

ascribed to a progressive increase in water depth, which was not large enough to

hinder wave winnowing of the tidal flat bottom (Fagherazzi et al., 2006), as

attested by storm-generated, sandy layers in the uppermost part of the muddy

succession (core 26 in Fig. 3.4). The constant wave reworking of the bottom of the

tidal flats in the Valle Millecampi area, causing suspension and removal of fine

sediments, is consistent with a maximum sediment accumulation of 1.30 m over

the last 1’600 – 1’900 years.

The correlation between sedimentary cores recovered from modern tidal

flats (see the Valle Millecampi areas in Figs. 3.2A, 3.2B) allows us to relate the

stratigraphic record (described for the first time in this work) to the historical

record derived from the detailed topographic map of Augusto Dénaix, (1809 –

1811, Fig. 3.9B). This map, which is the oldest map describing in details the

bathymetric characteristics of the Venice Lagoon (the older maps, e.g., Fig. 3.8 A,


78

Fig. 3.9. (A) Map of Cristoforo Sabbadino (1556). (B) Map of Augusto Dénaix, (1810). The red circle in the

two maps highlights the Valle Millecampi area (after D’Alpaos, 2010b, modified).

reported only qualitative information about the morphological characteristics of

salt marshes, tidal flats and channels), shows that most of the modern tidal flats,

presently cover areas which were previously colonized by salt marshes: i.e., the

width of the Valle Millecampi basin (sensu Mariotti and Fagherazzi, 2013) was

much smaller in 1810 than nowadays. Interestingly, none of the sediment cores

recovered in these tidal flats highlight the presence of salt-marsh deposits below

the wave-generated shell lag (Fig. 3.2B, core 26 in Fig.3.4). This suggests that these

deposits were entirely removed by waves, which promoted deepening of tidal

flats and lateral erosion of salt-marsh edges thus increasing the width of the Valle


79

Millecampi basin, at least, during the last two centuries. It is worth emphasizing

that, in this case, marshes did not disappeared due to drowning promoted by the

exceedance of a threshold rate of RSLR. On the contrary, the transition from a salt-

marsh to a tidal-flat landscape was triggered by the positive feedback between the

increase in wave power and the increase in the basin width and depth (Francalanci

et al., 2013; Mariotti and Fagherazzi, 2013). Larger fetches and increasing water

depths lead to increased wave height, wave power and lateral erosion, which

further promoted tidal-flat deepening and increase in the fetch, thus closing the

feedback. The extension of tidal-flat areas therefore occurred in parallel with the

decrease in salt-marsh extent, and the progressive water deepening led to bottom

elevations lower than MLWL and, consequently, to the conversion of tidal flats in

subtidal-platforms. The presence of typical tidal-flat deposits on the subtidal-

platform part of Valle Millecampi confirms the common origin of the sediments in

this area in which wind-wave erosion played a fundamental role on the marsh

platforms.

3.2.6.3 Depositional history of the area seaward of Punta Cane

Seaward of the Punta Cane salt marsh, the tidal flat which connects the

marsh with the subtidal platform of Fondo dei Sette Morti does not show a

succession similar to that observed for Valle Millecampi, but rather it presents a

salt-marsh succession similar to that observed for Punta Cane. The presence of

typical salt-marsh deposits which lie below MSL (core 7: elevation 20 cm below

MSL; core 29: elevation 8 cm below MSL; see Fig. 3.5) highlights that this seaward

part of the Punta Cane salt marsh is currently under lateral erosion. Day et al.

(1998b) found, by direct measurements in this area over 25 months, that the

seaward edge of Punta Cane salt marsh retreated at a rate of 0.6 ÷2.2 m yr-1. In this

area the action of the wind waves is particularly strong because of both its

exposure toward NE (Bora wind direction), the presence of quite a large fetch


80

(about 9 km of unlimited water surface between the Punta Cane salt marsh and

the littoral barrier) in the same direction, and the large water depths. As suggested

by Marani et al. (2011), the rate of edge erosion is linearly related to the incident

wave power density. Moreover, the erosion condition of the seaward part of the

marsh is also confirmed by the presence of a very steep scarp (much steeper than

the one which characterizes the sheltered portion landward of Punta Cane). As

suggested by Mariotti and Fagherazzi (2010), the scarp formation is a consequence

of the lowering of the tidal flat, which increases the height of the incoming waves

reaching the marsh edge and is responsible of an increase in marsh regression by

wave impact and bank failures (Francalanci et al., 2013), thus accelerating erosion.

3.2.7 Conclusions

In this study we provide the first accretion model of the latest Holocene

succession of the southern Venice Lagoon (transect through Valle Millecampi –

Punta Cane – Fondo dei Sette Morti). Our study succession testifies an evolution

from a palustrine freshwater environment to a lagoonal environment over the last

~2’000 years. The depositional history started with accumulation of peat deposits

in a deltaic setting which progressively evolved into a salt-marsh environment in

the 14th century. The aggradation of the salt-marsh system has been stemmed out

from both mud settling and organic accumulation, and occurred in parallel with

the decrease in the salt-marsh extent and the tidal-flat expansion. Indeed, where

salt-marsh deposits where locally flooded and impacted by wind waves, wave

erosion played a fundamental role, and a lag deposit developed. As a consequence

of the progressive water deepening, organic-rich mud accumulated above the lag,

originating tidal-flat deposits.

This study highlights also that the disappearance of salt marshes in this

area of the Venice Lagoon has to be ascribed to the lateral erosion of their margins

rather than to a progressive decrease in marsh elevations referred to MSL and the


81

following marsh drowning. Our observational evidence support the Mariotti and

Fagherazzi (2013) conceptual model based on the positive feedback between tidal

flat deepening and expansion and salt marsh retreat.

Acknowledgements

This work was supported by the CARIPARO Project titled “Reading

signatures of the past to predict the future: 1000 years of stratigraphic record as a

key for the future of the Venice Lagoon”, that is gratefully acknowledged.

Salt-marsh dynamics under high sediment delivery rates

83

CHAPTER 4

DYNAMICS OF SALT-MARSH LANDSCAPES UNDER

HIGH SEDIMENT DELIVERY RATES: THE CASE OF THE

SOUTHERN VENICE LAGOON

4.1 OVERVIEW

This chapter is a journal paper in preparation and it deals with the dynamic

response of a salt-marsh system to changes in sediment availability from a fluvial

source. The sedimentary succession of the Punta Cane salt marsh (southern Venice

Lagoon) was investigated through detailed sedimentological and elemental

analyses, together with the determination of the organic content and inorganic

grain size along the cores. The results were therefore wedged with the

chronological model proposed in Chapter 3, in order to detect the signatures of the

diversions of the Brenta River in/from the Lagoon.

4.2 INTRODUCTION

Tidal landforms are currently threatened by natural climate changes and

increasing human interferences, and are possibly subjected to potentially

irreversible transformations with far-reaching ecological, social and economic

implications worldwide (e.g., Costanza et al., 1997; Day et al., 2008; Barbier et al.,

2011). To address issues of conservation of tidal landforms, exposed to the effects

of climate changes and often to increasing human pressure, it is therefore of

critical importance to improve current understanding of the processes controlling

the response of these landforms to changes in the environmental forcings. This

issue is deemed of both intellectual as well as practical interest.


84

Tidal landscapes are shaped by physical and biological processes which act

over overlapping spatial and temporal scales, thus making the analysis of their

response to changes in the forcing a challenging task. Numerical modeling is an

effective tool to not only improve our theoretical understanding but also for

practical environmental management issues of tidal landscape evolution. A

number of models of morphodynamic evolution have been developed (e.g., Allen,

2000; Fagherazzi et al., 2012), which potentially offer a valuable tool to predict the

fate of tidal landforms: this is particularly interesting at sites which have been

exposed for centuries to natural climatic changes and severe human interventions.

However, the capability of existing models to provide a comprehensive and

predictive theory of tidal-landscape morphodynamic evolution seems to be

challenged by the incomplete understanding of the many linkages between the

relevant ecological and geomorphological processes (e.g., Murray et al. 2008;

Reinhardt et al., 2010). Moreover, in many cases, morphodynamic models resort to

the common approximation of a landscape in equilibrium with current forcings

and address predictions on future scenarios using the present observed

morphologies as an initial conditions (e.g., Allen, 1990; Kirwan et al., 2010).

Tidal environments are known to be highly dynamic systems in which salt

marshes, tidal flats, subtidal platforms and channel network represent a complex

intertwined system. Among the three unchanneled tidal sub-environments, salt

marshes represent a crucially important ecosystem due to their unique position in

the tidal frame (e.g., Mitsch and Gosselink, 2000; Chmura et al., 2003; Costanza et

al., 2008; MacKenzie and Dionne, 2008; Davy et al., 2009; Gedan et al., 2009;

Morgan et al., 2009; Perillo et al., 2009; Silliman et al., 2009; Barbier et al., 2011;

Gedan et al., 2011; Mcleod et al., 2011; Francalanci et al., 2013; Temmerman et al.,

2013; Möller et al., 2014). Effects of natural changes and human interferences on

these ecosystems are the responsible of possibly irreversible transformations,

which are manifested in a significant decrease in marsh extent worldwide,

especially during the last century (e.g., Day et al., 2000; Marani et al., 2003, 2007;


85

Carniello et al., 2009; Gedan et al., 2009; Mcleod et al., 2011). The rising sea level

and the lack of available sediments are suggested to be the key factors which

determine the drowning and disappearance of salt marshes worldwide (Day et al.,

2000; Morris et al., 2002; Reed, 2002; Marani et al., 2007; Gedan et al., 2009; Kirwan

et al., 2010; D’Alpaos et al., 2011; Mudd, 2011; D’Alpaos and Marani, 2015).

Salt marsh growth, development and maintenance occur through the

accretion rates provided by both inorganic and organic components, which

interact with each other and with the rate of sea level (e.g., Morris et al., 2002;

D’Alpaos et al., 2007; Marani et al., 2007; Li and Yang, 2009; Mudd et al., 2009;

Marani et al., 2010; Mudd et al., 2010; D’Alpaos 2011; D’Alpaos et al., 2011; Da Lio

et al., 2013; Marani et al., 2013). Marsh vertical growth within the tidal frame is a

function of the rates of inorganic and organic sedimentation, of the rate of sea-

level, and of the rate of sediment autocompaction (Allen, 1990; Bartholdy, 2012).

While inorganic and organic fractions and autocompaction terms are marsh-

elevation dependent (e.g., Allen, 2000; Mudd et al., 2009), only the relative sea-

level term is independent (Allen, 1990). As a consequence, measurements of sea-

level movements from salt-marsh accretion rates could be not authentic and they

could be characterized by a lag effect in which measured accretion rates either

over-estimate or under-estimate the rate of sea-level changes (Allen, 1990). It has

recently been observed, through mathematical modelling, that marsh morphology,

and its effect on biological productivity and vertical accretion, could lag century-

scale sea-level rise rate oscillations by several decades (Kirwan and Murray, 2005,

2008). As a consequence, salt marshes might not have been in equilibrium with the

Holocene sea level and they might currently be out of equilibrium with modern

rates of sea level rise, reflecting environmental conditions of the past (Kirwan and

Murray, 2008). Kirwan and Temmerman (2009), on the basis of a point model,

observed that marsh elevations adjust to a step change in the rate of sea-level rise

in about 100 years (although this needs to depend also on sediment availability),

but in the case of a continuous acceleration in the rate of sea-level rise, modeled


86

accretion rates lag behind sea-level rise rates by about 20 years, and never obtain

equilibrium. However, changing rates of sea-level rise may not be the dominant

factor determining the evolution/degradation of some marshes (Kirwan and

Temmerman, 2009; Kirwan et al., 2011). D’Alpaos et al. (2011) showed, on the basis

of an analytical point model, that the time-lag characterizing the response of

marshes to perturbations in the environmental forcings depends not only on RSL

oscillations but also on sediment availability and tidal range. Moreover, marshes

are more resilient to a decrease rather than an increase both in the rate of RSLR

and in the sediment availability. As a consequence, a lag has also to be expected

between changes in sediment availability and new equilibrium conditions, in

analogy to the case of changes in the rate of RSLR. Kirwan et al. (2011) document

an example of rapid salt marsh expansion during the 18th and 19th centuries due to

increased rates of sediment delivery following deforestation associated with

European settlement in the Plum Island Estuary (Massachusetts, US). The authors

also suggest that current marsh degradation in the North America coast may

represent a slow return to pre-settlement marsh extent with a natural state.

The Venice Lagoon, as observed in other lagoons and tidal environments

worldwide, is currently threated by a severe decrease in salt-marsh area, together

with a general expansion and deepening of tidal flats and subtidal platforms. Salt-

marsh areas decreased from about 255 km2 in 1611 to about 180 km2 in 1810. The

decrease peaked in the last century when the marsh areas decreased to about 50

km2 in 2002, with a reduction of more than 70% compared to marsh extent in 1810

(Marani et al., 2003, 2007; Carniello et al., 2009; D’Alpaos, 2010a). Moreover,

marshes might not have yet fully responded also to historical decrease in sediment

supply and further adjustments in the form of decreasing marsh-platform

elevation could be expected (D’Alpaos et al., 2011).

In the Venice Lagoon, the most severe changes in terms of sediment supply,

together with large amounts of freshwater inputs, occurred in the last millennium

due essentially to the diversion, carried out by the “Serenissima” (as the Republic


87

of Venice used to call itself), of the main rivers which used to debouch into the

lagoon. These measures were undertaken to avoid the infilling of the Lagoon with

sediments. An outstanding number of sedimentological and modelling

investigations has been carried out in the Venice Lagoon aiming at defining the

stratigraphy of the Holocene sedimentary succession (e.g., Madricardo et al., 2007;

Tosi et al., 2007a, 2007b; Brancolini et al., 2008; Zecchin et al., 2008; Tosi et al.,

2009a; Zecchin et al., 2009, 2011; Madricardo and Donnici, 2014; Zecchin et al.,

2014) and the morphodynamic changes of the modern lagoonal environment (e.g.,

Marani et al., 2007; Carniello et al., 2009; Amos et al., 2010). However, the higher

sedimentological resolution focused mainly on environmental changes occurred at

the millennium scale (e.g., McClennen and Housley, 2006; Madricardo et al., 2007;

Tosi et al., 2007a, 2007b; Brancolini et al., 2008; Zecchin et al., 2008, 2009, 2011;

Madricardo and Donnici, 2014), leaving a major gap regarding the past 1,000

years, which were characterized by the highest human interventions on the

lagoonal system. The knowledge of the response of tidal environments to changes

in the forcings at centennial – decadal scale is therefore an important and critical

issue for the determination of a detailed evolution of these systems.

In this work we focus on the response of a salt-marsh system in the Venice

Lagoon, located in an area deeply involved in sediment supply and freshwater

input changes during the last millennium because of the repeated diversion of the

Brenta River system into the lagoon. The goal of this study is twofold: i) to detect

the signature of the Brenta River in the Punta Cane salt-marsh sedimentological

succession through a multidisciplinary approach based on the integration between

sedimentological, stratigraphical, geochronological, and elemental data; ii) to

verify through field measurements and analyses the existence of a time-lag

between the flowing of the Brenta River into the lagoon and the morphodynamic

response of the Punta Cane salt-marsh system.


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4.3 GEOLOGICAL AND GEOMORPHOLOGICAL SETTING

4.3.1 The southern Venice Lagoon

The Venice lagoon is an elongated and arched waterbody located in the

northwestern Adriatic Sea (Fig. 4.1A). It is the largest lagoon in the Mediterranean

with an area of about 550 km2, a mean water depth of 1.5 m, and a semi-diurnal

micro-tidal regime (maximum water excursion at the inlets of ± 70 cm around

MSL). The three inlets of Lido, Malamocco and Chioggia permit the exchange of

water and sediments with the Adriatic Sea.

The Venice Lagoon originated by flooding of the upper Adriatic plain due

to rising sea levels that followed the Last Glacial Maximum, and formed over the

last 7’000 – 6’000 years. The formation of the Venice Lagoon occurred

diachronically, starting from the south, where the oldest sediments are dated at

about 7’000 years ago. In the central part of the lagoon, sedimentation started

around 6’000 years ago (Favero and Serandrei Barbero, 1978, 1980).

Fig. 4.1. (A) Location of the study site in the southern Venice Lagoon. (B) Punta Cane salt marsh

area and current path of the Brenta River. (C) Location of the three sediment cores, object of this

study, in the Punta Cane area.

The southern portion of the Venice Lagoon contains a ~20 m thick Holocene

sedimentary succession (Tosi et al., 2007a, 2007b; Brancolini et al., 2008; Zecchin et

al., 2008, 2009) which overlays Pleistocene continental deposits accumulated


89

during the Last Glacial Maximum (LGM). The Holocene succession is composed

of three main seismic units, separated by key stratal surfaces (Zecchin et al., 2009).

The surface topping the LGM deposits is an unconformity surface generated by

fluvial incision. Accordingly, the Holocene sedimentary succession is made of

three main units. The first unit consists of incised valley fills passing upward into

lagoonal deposits which are abruptly overlain by shoreline deposits, associated

with the maximum marine ingression occurred at around 6’000 years BP. During

this ingression, the shoreline shifted from 4 km (Fondo dei Sette Morti area) to up

to 15 km (Adige River area) landward (Favero and Serandrei Barbero, 1980). The

second Holocene unit represents the regressive phase started after the maximum

marine ingression. It consists of delta plain, swamps and marshes deposits

landward, and prodelta, shoreface and beach ridge deposits seaward. At this

stage, sediment supply from the mainland caused the seaward shift of the

shoreline, which reached the current position around 2’500 years BP (Favero and

Serandrei Barbero, 1980). The irregular surface which marks the passage from the

second to the third Holocene unit is younger than 2’000 years and corresponds to

the base of the recent lagoonal deposits. It represents the erosive base of

channelized deposits and, outside of the channels, to a surfaces which marks an

abrupt change in the depositional style. The uppermost Holocene unit is, in fact,

associated to the deposition of tidal-channel, tidal-flat, subtidal-platform deposits

that follow the generalized drowning of the southern part of the Venice area, and

a parallel remarkable change in the hydrodynamics, primarily due to human

interventions (Zecchin et al., 2009)

4.3.2 The Brenta River

The Late Pleistocene – Holocene evolution of the southern part of the

Venice Lagoon (Fig. 4.1B) is strongly influenced by the sediment and water supply

from the Brenta River, especially during the Latest Holocene, when the river was


90

repeatedly diverted in/from this area. The Brenta River is one of the major

watercourses draining the Dolomites (Southern Alps). It has origin from

Caldonazzo lake (south-eastern part of Trentino-Alto Adige region) and flows into

the Adriatic sea, northern of the Po River, at Brondolo mouth, although a minor

small reach (the Naviglio Brenta) is still active and flows towards the central part

of the Venice Lagoon at Fusina mouth. The Brenta River has a length of 174 km

and a drainage basin of 1567 km2. Its mean annual discharge is about 71 m3 s-1,

while the flood peak discharge is 2400 m3 s-1 (Surian et al., 2009). As for the

hydrological characteristics of its watershed, the mean annual precipitation is 1313

mm and runoff at the basin outlet is 105%. This high value of runoff is due to the

contribution of karst springs, which are located in the lower part of the drainage

basin (Surian and Cisotto, 2007). The course of the Brenta River can be divided

into two reaches: an upper reach, 70 km long, where the river flows within the

mountain area, and a lower reach, 104 km long, where it flows in the Venetian

Plain. In the mountain area, the Brenta River collects water from the Cismon creek,

which is its main tributary. These two watercourses drain two catchment basins of

about 650 km2 each. Along more than half of its course, from the source up to the

Upper Venetian Plain, the Brenta River channel bed is composed of coarse

material, whereas sand and silt are found in the lowest reach. Different types of

rock crop out in the drainage basin: in the upper part of the basin, gneiss, phyllite,

granite and volcanic rocks (andesite, rhyolite, etc.) and subordinate limestone and

dolomite are found, whereas limestone and dolomite are predominant in the

lower part (Surian and Cisotto, 2007).

The current lower course of the Brenta River (from the Padova area to the

mouth) has been strongly modified by anthropic interventions, being the result of

a series of diversions and hydraulic arrangements carried out over the last

millennium. During 16th and 17th centuries, the Republic of Venice carried out

repeated diversions of watercourses draining into the Venice Lagoon. River-fed

deposits settled into the lagoon causing shallowing of ship canals and,


91

consequently, a status of economic and environmental crisis, that forced the

Republic of Venice to start a massive program of artificial river diversions which

brought to strong changes in the paths of several rivers, such as the Brenta, Po,

Bacchiglione, Adige, Sile, Piave and Livenza (Bondesan and Furlanetto, 2012).

Historical sources (e.g., Mozzi et al., 2004; Primon and Furlanetto, 2004; D’Alpaos,

2010a; Bondesan and Furlanetto, 2012) suggest 1143 AD as the moment in which

the Brenta River was diverted along the current riverbed of the Naviglio Brenta,

with two probable mouths: the first one at Fusina (the current mouth of the

Naviglio Brenta), and the second one around Santa Marta, where currently the

industrial area of Porto Marghera is located (Fig. 4.2A). In 1457 AD the Venetians

diverted southward the downstream reach of the Brenta riverbed in the artificial

“Diversivo di San Bruson” channel (Fig. 4.2A). With the aim of moving away the

River from Venice city, in 1507 AD a new canal was excavated. In the area of

Conche, the Brenta River was joined with the Bacchiglione River, and the two

rivers flowed in the southern portion of the lagoon flowing through the

“Montalbano channel” (Fig. 4.2B). In 1540 AD the outlet of the Brenta-Bacchiglione

system was moved directly in the Adriatic sea in the Brondolo area. In this frame,

the two riverbeds were divided in the area of Conche, and connected again

downstream, about 5 km before the Brondolo mouth (Fig. 4.2C). To ensure a better

drainage, the Brenta River was shifted in 1610 AD in the “Taglio Nuovissimo”, in

which the river flowed along the boundary between the mainland and the lagoon

(the so-called “Argine di conterminazione lagunare”), which represents the Venice

Lagoon boundary lines defined by the Republic of Venice (Fig. 4.2D). The Brenta

River maintained the last configuration until 1840 AD when, to increase the slope

of the river by the shortening of the riverbed, its outlet was shifted again toward

the area Conche. From 1858 AD to 1896 AD, the Brenta River, divided from the

Bacchiglione River, flowed in the “Cunetta del Brenta”. It is worth noting that in

these 38 years the Brenta River provided a widespread sediment accumulation in

its delta. The further reclamation of the delta led to the construction of an area of


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Fig. 4.2. Evolution of the Brenta River over the last millennium. (A) Brenta River in 1143 AD

with the two mouths at Santa Marta and Fusina; Brenta River diversion from 1457 to 1507 AD.

(B) Diversion of the river in 1507 AD. The Brenta and Bacchiglione flowed together in the

southern portion of the Venice Lagoon. (C) Outlet of the Brenta-Bacchiglione system at the


93

Brondolo mouth from 1540 to 1840 AD. (D) Diversion of the Brenta River in 1610 AD. (E) Outlet

of the Brenta River in the southern part of the Venice Lagoon from 1858 to 1896 AD. (F) Current

configuration (after D’Alpaos, 2010a, modified).

27 km2 which is nowadays part of the mainland (the so-called “Bonifica Delta

Brenta”) (Fig. 4.2E). At the end of the 19th century, the outlet of the Brenta River

was definitively again moved to the Brondolo mouth, in which the Brenta-

Bacchiglione system currently flows into the Adriatic sea. The “Naviglio Brenta”,

which represents one of the two small reaches of the first configuration of the

Brenta River, is active and flows into the lagoon southern of Venice city (Fig. 4.2F)

at the Fusina mouth (e.g., D’Alpaos 2010a, 2010b).

4.3.3 The study site

The study site is located in the southern part of the Venice Lagoon, in the

Punta Cane area (Figs. 4.1B, 4.1C). A recent stratigraphical and geochronological

study on a sedimentary succession in the southern part of the Venice Lagoon

(Chapter 3 on this thesis) was performed by collecting 25 sediment cores

distributed along a linear transect 5.2 km long cutting through subtidal-platform,

tidal-flats and salt-marsh deposits and involving Valle Millecampi, Punta Cane

and Fondo dei Sette Morti areas. (Fig. 4.3A). For the Punta Cane zone, that study

highlighted the presence of a sedimentary succession of about 3.50 m in thickness

accumulated over the past two millennia (Figs. 4.3A, 4.3B). This succession

consists of a basal peaty body accumulated in a palustrine environment, which

was already active during the 1st century BC. Palustrine sedimentation persisted

until about the 14th century allowing the accumulation of about 2 m of peat.

Around 1355−45+40 cal AD the palustrine environment changed into a salt marsh

suggesting the establishment of brackish water conditions and an increase in

muddy sediment supply. Salt-marsh deposition persisted until the present day

bringing to an accumulation of ~1.80 m of sediment in about 650 years. Detailed


94

radiocarbon and radionuclides (210Pb and 137Cs) datings on the salt-marsh

succession (Fig. 4.3B) show a non-constant accumulation rate over time, with

values ranging from 0.09 cm/year to up to 1.18 cm/year.

Fig. 4.3. (A) Sedimentological and stratigraphical interpretation of Valle Millecampi, Punta

Cane, Fondo dei Sette Morti areas. Cores 1, 29, 28 represent the three sediment cores involved in

the present study. (B) Geochronological data in the Punta Cane area. Each calibrated age is

expressed as the mode value in the distribution and the two errors represent the ages interval at

68% of probability.

4.4 MATERIALS AND METHODS

To analyze the response of the salt-marsh system to changes in the

sediment load provided by inputs of the Brenta River over the last ~1’000 years,

we performed a series of analyses on three cores (1, 29, 28 in Figs. 4.3A, 4.3B). To

detect changes along the succession, high resolution sedimentological analysis,

determination of the organic matter content, detection of the inorganic fraction

grain size and elemental variations were analyzed.


95

4.4.1 Sedimentological analysis

A high-resolution sedimentological analysis was carried out on the three

study cores following the principles of modern facies analysis. This analysis aimed

at defining the sedimentary distinctive features of different types of deposits and

to link them with corresponding depositional environments. Deposits were

differentiated on the basis of their color, grain size, texture, and sedimentary

structures. Macroscopic biogenic content, mainly consisting of shells, plant debris

and in situ vegetal remains, was also noted.

4.4.2 Determination of the organic fraction

The analysis of the organic matter content was performed on cores 1 and 29

by Loss On Ignition (LOI) at 375°C for 16 hours, on a total number of 62 samples

belonging to core 1 (36 samples) and core 29 (26 samples), collected every 3 cm

from the top of the cores. A total amount of 1.20 ± 0.20 g of dry sediment for each

sample, dried at 60°C for 36 hours, was used to perform LOI analyses. The

sediment was crumbled in a ceramic mortar and placed in a dry ceramic crucible.

The LOI process started with a temperature increase of 5°C/min until reaching

375°C, and continued at a constant temperature for 16 hours (Ball, 1964;

Frangipane et al., 2009; Protocol of SFU Soil Science Lab, 2011). The difference in

weight, before and after the burning process, was used to estimate the amount of

organic matter which was combusted. The organic content will be expressed as a

percentage of the organic matter.

4.4.3 Particle size analysis

The particle size analysis was performed on the residual inorganic sediment

fraction obtained after a chemical treatment with hydrogen peroxide (H2O2),

according to Gray et al. (2010), to remove the organic component. As for the


96

determination of the organic matter, a total number of 62 samples, corresponding

to the same stratigraphic layers used for LOI analyses (cores 1 and 29), were

analyzed. Each sample, characterized by a dry weight of about 6 g, dried at 60°C

for 36 hours, was treated with 35% H2O2 for 36 hours. At the end of the oxidation,

when no visible frothing occurred, dilution with deionized water, decantation (for

24 hours), siphoning, and drying were carried out. Deionized water was added to

each sample to obtain a dispersed particulate sample. The particle size analysis

was carried out using a Mastersizer 2000 (Version 5.40, MALVERN

INSTRUMENTS). The Mastersizer 2000 uses laser diffraction to measure the size

of particles, by measuring the intensity of light scattered as a laser beam passes

through the dispersed particulate sample. These data are then analyzed to

calculate the size of the particles that created the scattering pattern. The grain size

results will be presented as D50 distribution.

4.4.4 X-Ray Fluorescence

X-Ray Fluorescence (XRF) is a rapid and non-destructive analytical

technique for the determination of the chemical composition in a material. Our

XRF analysis was carried out on core 1 at ETH Zurich using an AVAATECH XRF

core scanner, which is an energy dispersive XRF. The elemental range determined

with the AVAATECH XRF core scanner starts from Aluminum (atomic number

13) to Uranium (atomic number 92) in concentrations from 100% down to ppm

levels. In general, elements with higher atomic numbers have lower detection

limits than lighter elements. Elements with a smaller atomic number are not

traceable given that the X-ray radiation of the lighter elements is easily absorbed

and therewith not able to enter the detector. The response depth of elements for

incoming X-rays depends on the wavelength of emitted X-rays, the rank in the

period system and the chemical composition of the sample matrix increases with

increasing atomic number, as for example Al: 0.05 mm; Ca: 0.5 mm; Fe: 1 mm; Ba:


97

2-4 mm for average marine sediments (Jenkins and De Vries, 1970; Richter et al.,

2006).

The XRF core scanner at ETH Zurich is equipped with a variable masking

slit system that prevents detection of radiation from beyond the analytical area,

resulting in resolution between 10 and 0.1 mm down-core and 15 to 2 mm cross-

core. The optical system with the slit system has contact with the sediment surface

during measurements and is flushed with Helium for a proper detection of the

lighter elements. The sample, whose surface has to be smoothed as much as

possible, is covered with a 4 μm thin foil “Ultralene” to prevent drying out of the

sediment, the diversion of sediments and to protect the sample against dirt. The

source of the radiation is an Oxford 100 Watt water-cooled X-ray tube with a 125

μm Beryllium window. The digital Canberra X-ray detector with Beryllium

window contains a 1.5 mm thick Si-crystal for a better detection of elements > As.

The measurement time depends strongly on the down-core resolution and the

measuring time per sample. For instance, a sediment core with a length of 1 m

measured in standard setting with the down-core resolution of 1 mm for all

elements takes a time of 24 hours.

The XRF measurements were carried out on core 1 at 10, 30, 50 kV and at a

resolution of 2 and 10 mm down-core. A digital Jai CV L105 3 CCD Color Line

Scan Camera records images with a resolution of 140 ppcm (350 dpi, 70 μm).

4.5 RESULTS

4.5.1 Sedimentological analysis

The sedimentary succession is documented by a composite core obtained

from the physical correlation between cores 1, 29 and 28 (Fig. 4.4). The composite

core is 2.76 m thick, and its top and base are located 19 cm and 257 cm above and

below the MSL, respectively. The sedimentary succession starts with 70 cm of


98

massive peat with abundant fragments of reeds. The peat consists of comminuted

plant debris with a minimum amount of dispersed mud and very fine sand. This

deposit formed in a palustrine environment in which vegetal remains were

transported as debris or produced in situ.

The palustrine peat grades upward into salt-marsh deposits. This transition

occurs within ~20 cm and is characterized by a gradual decrease in the reed-

fragment content and an increase in the mud settling. Finer reeds fragments and

plant debris are still present in the lowermost part of the salt-marsh deposit. The

salt-marsh deposit is about 1.85 m thick and consists of a horizontally-laminated,

bioturbated, brownish mud, with a variable amount of fine to very fine sand. Dark

brown mud layers are common and are very rich in plant debris and roots. The

latter are common in the uppermost part of the core. Light grayish very fine mud

intervals are less common. They are about 15 cm thick and commonly contain

scarce vegetal remains (e.g., intervals from 45 cm to 60 cm and from 95 cm to 110

cm down-core). Sand is mainly organized in millimetric, whitish, horizontal

laminae which are characterized by a good grain size sorting.

The accumulation of salt-marsh deposits occurs in the highest portion of the

intertidal range, where the organic matter produced by halophytic vegetation

contributes to the marsh accretion together with the inorganic component. Mud

settles down around high water slack, at the transition between flood and ebb

tides, while the sandy laminae are generated during storm events during which

waves can re-suspend sand and mud from the tidal flats and subtidal platforms in

front of the marsh and deliver them onto the marsh platform. In addition, when

wind-generated waves winnow the salt-marsh surface, they further suspend mud

thus concentrating sand.


99

Fig. 4.4. Composite core obtained from the physical correlation between cores 1, 29, 28.

Thickness, sedimentological log and main features of the deposits are shown.


100

4.5.2 The organic fraction

The organic matter content determined by LOI along the two studied cores

(Fig. 4.5) shows a variable distribution between 8.5% and 36.1%; these percentages

calculated for each sample as lost weight after burning at 375°C for 16 hours.

As to core 1 the analysis starts at 7 cm from the top of the core because the

uppermost sedimentary succession was disturbed during the core recovering. As a

whole, the organic content gradually decreases from the top of the core toward the

bottom, with an average percentage of organic matter equal to 15.5%. More in

detail, a first decreasing trend can be observed from the top up to 86÷90 cm below

MSL, with value ranging between 19.9% (at 7 cm above MSL, maximum organic

matter value along the core) and 8.5%, where the organic matter content reaches

the minimum values. After this interval, the organic content increases again. A

second marginally decreasing trend can be observed from a depth of 93 cm below

MSL to the bottom of the core, with values ranging between 17.8% and 14.8%.

A different situation can be observed for the core 29 (Fig. 4.5). As a whole,

the organic content tends to increase from the top of the core toward the bottom,

with values ranging between 14.5% and 36.1% and an average percentage value of

20.1%. The first meter of the core shows organic values comparable with those of

core 1 (minimum value: 14.5%; maximum value: 22.5%), with variable

increasing/decreasing trends. At about 1.30 m below MSL the organic matter

undergoes an important increase, with a percentage of organic content increasing

from 19.5% to 36.1% at about 1.50 m in depth. The sample at the bottom of core 29

(1.58 m below MSL) shows instead a clear decrease, reaching a value of 26.3% of

organic matter.


101

Fig. 4.5. Organic fraction content (blue lines) and D50 grain size (red lines) distributions for cores

1 and 29.

4.5.3 Particle size analysis

As to core 1, the grain size values for the inorganic sediment fraction range

between 724 µm (maximum value) and 0.72 µm (minimum value). On average,

the grain size shows a variable distribution between a medium sand (average

coarser value: 360 µm) and a clay (average finer value: 0.72 µm). The median grain

size D50 (Fig. 4.5) is in the range 15÷50 µm and shows an oscillating trend from the

top of the core toward the bottom highlighting an alternation of finer and coarser

interval.

As to core 29 the grain size values for the inorganic sediment fraction range

between 2.19 mm (maximum value belonging to the field of granules) and 0.72 µm

(minimum value). The median D50 distribution (Fig. 4.5) ranges between 19 µm

and 168 µm. The distribution can be however divided in two intervals, determined

on the base of an abrupt trend change found at about 1.30 m below MSL. The

upper interval, from the top of the core to about 1.30 cm below MSL, does not

show any significant change in grain size, with values ranging between 19 µm and

50 µm. The lower interval shows D50 distribution ranging from 43 µm and 168 µm


102

and highlights a clear decrease in grain size. The grain size results of the upper

interval show clear affinity with those obtained for core 1. The Mastersizer, in fact,

recorded a variable inorganic fraction distribution between medium sand (average

coarser value: 431 µm) and clay (average finer value: 0.68 µm). On the contrary,

for the lowermost interval of core 29, the values increase of more than one order of

magnitude, with grain size values distributed between a very coarse sand (average

coarser value: 1270 µm) and clay (average finer value: 0.84 µm).

4.5.4 X-ray Fluorescence

The results of XRF analysis are summarized in Fig. 4.6, where distribution

of Si, Al, K, Fe along the core 1 was obtained at 10, 30, 50 kV and with a resolution

of 10 mm down-core. Silica, aluminum, potassium and iron are typical elements in

clay minerals, in feldspars and in zeolites (except for Fe), silica is the main element

in quartz. Overall, the XRF trends oscillate. The signal appears to be disturbed in

the first 10 cm from the top, probably due to the sediment re-working during

sampling. Negative peaks which occurs, for example, at about 30 cm, 60 cm, 90cm

and 120 cm in depth, can possibly be ascribed to the presence of small cracks

generated by drying of the core. The XRF signal however highlights the presence

of two intervals in which the concentration of the above-mentioned element

reaches defined positive peaks. From the top of the core, the first peak is generated

by a 10 cm thick layer that occurs between 45 cm and 55 cm down-core. The

second interval (which again is about 10 cm thick) is more evident and occurs

between 1.0 m and ~1.10 m down-core.


103

Fig. 4.6. XRF results for core 1. Silica, aluminum, potassium and iron trends along the core are

reported. The two red stripes highlight the intervals in which the above-mentioned elements

reach defined positive peaks.

4.6 DISCUSSION

Organic matter content and grain size analyses on cores 1 and 29 do not

appear be, individually, useful indicators for determining the signature of a high

sediment delivery in the Punta cane salt-marsh succession (Fig. 4.5).

The analysis of the organic content does not show significant changes along

the two considered cores. However, the slightly decreasing trend in core 1

highlights the presence of a ~11 cm-thick interval (from -83 to -94 cm referred to

MSL) in which the lowermost values occur. The interval corresponds to a grayish

layer in the sedimentary core (Fig. 4.4) made of fine mud in which the organic

material is very scattered. The organic-content analysis together with the

sedimentary features of this interval support the hypothesis of a short-time change

in the depositional features, in which the inorganic component largely prevails on

the organic one. In core 29 instead, the oscillatory trend does not show any

significant change until about 1.30 m below MSL, depth at which an important

increase in the organic matter content occurs. This behavior is explained by the


104

fact that the lower part of core 29 is very close to the transition between the basal

palustrine peat and the salt-marsh deposit which is marked by a decrease in the

reed fragments, typical of the palustrine sediments, and by an increase in the

inorganic fraction, fundamental component for marshes accretion. The high values

in organic matter content for the lower 20 cm of core 29 represent therefore the

influence of the underlying deposit, rather than an important increase in the salt-

marsh organic deposition. No evidence of changes in fluvial sediment input are

therefore observed.

As for the grain size of the inorganic fraction, the oscillatory trend of core 1

and the nearly vertical trend of core 29 up to the depth of about 1.40 m below

MSL, do not show significant changes suggesting the presence of an important

sediment supply possibly provided by the Brenta River debouching in the

southern Venice Lagoon. The occurrence of small positive peaks in the grain size

distribution of the inorganic fraction, corresponds to the presence of sandy

laminae which, during storm events and high tide conditions, were accumulated

on the salt-marsh surface by wind-generated waves. The remarkable increase at

the bottom of core 29 should not be misinterpreted. Such an increase can indeed be

ascribed to the presence of organic matter in the disperse particulate samples

undergoing the grain size analysis (which was still visible with the unaided eye),

rather than to a real increase in the size of the inorganic fraction. In fact, it is

generally agreed that the H2O2 chemical treatment cannot totally remove the

organic fraction in samples characterized by a high presence of organic matter

(e.g., Mikutta et al., 2005), as in the case of the samples at hand. In this way, if the

Mastersizer laser beam intercepts the organic grain along its commonly elongated

shape, the scattering pattern records a grain size larger than the real one, thus

providing a distorted estimate of the grain size distribution. This hypothesis is

also confirmed by the results of the organic matter analysis, which highlight an

increase in the organic component in the lowermost portion of core 29 because of

its closeness to the underlying peaty deposits. The fact that there are no significant


105

visible intervals in which negative or positive peaks in organic content and size of

the inorganic fraction, is quite a surprising result. No signatures of the sediment

inputs provided by the Brenta River can be detected. However, one would expect

the presence of a large sediment delivery provided by the Brenta River, when

debouching into the lagoon, to be clearly reflected in the sedimentary succession.

It is however worthwhile observing that the distance between the outlet of the

river and the Punta Cane area (~5 km) is probably the main cause for the absence

of the signatures we expected to find. We deem it can hardly be denied that the

Brenta River flowing into the Venice Lagoon provided a large sediment supply. It

is, in fact, well known, also from historical maps, that during its last diversion into

the lagoon from 1858 to 1896 AD, the Brenta River led to an exceptional sediment

accumulation around its debouching area. This area in fact has been reclaimed at

the beginning of 20th century and is nowadays part of the mainland (the so-called

“Bonifica Delta Brenta”). XRF results appear to be the most important proxy, or at

least the clearest, for the determination of well-defined intervals which can

represent the signature of the Brenta River system flowing into the Venice lagoon

(Fig. 4.6). The occurrence of intervals with a high concentration of silica,

aluminum, potassium and iron is consistent with the lito-types composition

outcropping the Brenta River drainage basin, especially in its upper mountainous

one.

However, the multidisciplinary approach provided results, in terms of

organic content, grain size, XRF and sedimentological analysis, which wedged

with the chronological model, provide an interesting output (Fig. 4.7). The red

intervals in the figure represent the two XRF positive peaks. They correspond both

to finer intervals in the sedimentary succession of core 1, only partially showed in

the D50 trend, and to intervals in which the organic matter content in the salt-

marsh succession is low (the lowermost XRF interval corresponds to the

lowermost values in organic content in core 1). The chronological model shows a

salt-marsh evolution during the last 650 years, and, consequently, the three Brenta


106

Fig

. 4.7. Resu

lts of th

e mu

ltidiscip

linary

app

roach

for th

e Pu

nta C

ane salt m

arsh su

ccession

.


107

River inputs into the lagoon should be recorded in the sedimentary succession.

The first XRF interval, from about 80 cm to 90 cm above MSL, is located just before

1640−60+20 cal AD, while the uppermost one, from about 25 to 35 cm, is just after

1895−9+13 cal AD. It is clear that the red intervals, wedged with the chronological

model, cannot represent the presence of the Brenta River system because of their

younger ages in respect to the effective time-spans of the river flowing into the

Lagoon. In fact, based on the chronological model, the blue intervals in Fig. 4.7

ideally represent the timing in which the Brenta River system flowed into the

lagoon (1457 – 1507 AD; 1507 – 1540 AD; 1858 – 1896 AD; Figs. 4.2A, 4.2B, 4.2E).

This multidisciplinary approach highlights the occurrence of a temporal lag

between the presence of river water and sediment fluxes into the lagoon, and the

evidence of their signature in the sedimentary succession. In this case, we interpret

the temporal lag as the consequence of a temporary storage of the high sediment

delivery provided by the River. These sediments are temporary stocked within the

basin and only at later stages they are re-suspended by waves, redistributed by

tidal currents and delivered onto the salt-marsh surface, where they can settle. The

accumulation of the stocked sediments on the marsh surface, increases marsh

elevation relative to MSL, and promotes a decreasing of organic matter production

(provided by the halophytic plants which populate the marsh) which depends on

marsh elevation within the tidal frame. This leads to a lower organic accumulation

in the sedimentary succession.

In addition, the lack of signatures of large sediment input in the

sedimentary record of the considered transect can be interpreted by considering

that the main effect of the sediment pulse is to promote marsh horizontal

progradation, rather than of increasing marsh elevation within the tidal frame. The

Brenta River was reintroduced into the Lagoon for about 40 years (between 1858

and 1896 AD), during which it promoted the formation of 27 km2 of marshes, now

reclaimed. Previous modelling efforts (D’Alpaos et al., 2011) suggested the

existence of a lag between the presence of a perturbation in the forcing (i.e., a high


108

sediment pulse), and the response of a salt-marsh system to the change in the

forcing itself. Although our results seem to suggest the existence of a lag of this

type, it is worth noting that the recalled modelling approach is based on a zero-

dimensional approximation, i.e., that model considers one point as representative

of the whole marsh platform. In this very case the space dependent dynamics of

the considered system seems to be a relevant role in the relaxation time required

for the system to reach new equilibrium conditions. The source of sediment input,

in this case, is about 5 km apart from the Punta Cane marsh, and this might have

an important effect.

In Fig. 4.7 is also interesting to note that during the flowing of the Brenta

River into the Venice Lagoon, the accretion rates of Punta Cane salt marsh do not

show significant high values. The highest accumulation rate values occur after the

presence of the river.

4.7 CONCLUSIONS

We analyzed the possible response of a salt-marsh system in the southern

Venice Lagoon to changes in sediment delivery rates provided by the Brenta River

system. The repeated diversion of the river into the lagoon influenced the

evolution of the Punta Cane salt marsh. In order to detect the signature of the

presence of the Brenta River system in the sedimentary succession,

sedimentological and elemental analyses were carried out, together with the

determination of the organic content and grain size of the inorganic fraction along

cores.

The results from our analyses suggest that a multidisciplinary approach is

required in this case, any of the different analysis being sufficient alone to detect

and interpret important changes in a sedimentary succession.

Sedimentological, elemental, organic content and grain size analyses,

wedged with a chronological model of the Punta Cane salt marsh, show that the


109

marsh response to changes in sediment delivery rates is not instantaneous. A

temporal lag exists between the presence of the Brenta River system into the

lagoon and its identification in the salt-marsh succession. The signature of the

river along the salt-marsh sequence appears with finer inorganic deposition,

decreasing of organic matter accumulation and positive XRF peaks in elements

which are consistent with the outcrops composition of the Brenta River drainage

basin.

Conclusions

111

CHAPTER 5

CONCLUSIONS

In this work, a series of field and laboratory analyses allowed us to improve

our knowledge of the biomorphodynamic evolution of salt-marsh systems. In

particular, the work contributed to improve our understanding of both the

evolution of salt-marsh bio-geomorphological patterns and the relative

importance of physical and biological processes, and the response of salt-marsh

systems to changing in the environmental forcings.

The high-resolution spatial analyses on modern sub-surface marsh

sediments from the northern Venice Lagoon, unraveled the variations and the

mutual feedbacks between the physical and biological processes at the marsh

scale. The results emphasize that surface elevations, inorganic and organic

sediment content, and grain size distribution along marsh transects are tightly

related. In particular, higher elevations and coarser sediments are found along

marsh edges, while the inner portion of the marsh, characterized by lower

elevations, is reached only by finer sediments, suggesting that the tidal network

which cuts through the tidal landscape largely controls inorganic sediment

transport over the platform. In addition, the results highlight that the

determination of soil organic matter and inorganic content is very sensitive to the

specific analysis method (LOI, H2O2, H2O2 + NaClO), bearing important

implications in determining the real values of the marsh soil organic content, the

stocked organic carbon and the organic and inorganic accumulation rates. The

position on the salt marsh and the distance from the channels strongly affect the

rates of accumulation of the organic and inorganic components: accretion is

mainly driven by the inorganic component in proximity of the channels, whereas

the organic component becomes important in the inner part of the marsh.

Conclusions

112

Moreover, the accumulation of organic matter in the soil does not increase with

biomass production, which is generally higher along the channel banks and lower

in the inner part of the marsh. Organic matter accumulation in the soil is governed

by the interplay of plant biomass productivity and decomposition. This interplay

is, in turn, modulated by soil aeration, favoring decomposition, which is highest

near the marsh edge and lowest in the low-lying inner zones, where the reduced

decomposition rate compensates the lower biomass productivity. The analyses

also provided the first estimates of soil organic carbon density and of the carbon

accumulation rate (132 g C m-2 yr-1) for marshes in the Venice Lagoon.

The multidisciplinary analyses carried out on the latest Holocene

sedimentary succession in the Punta Cane area, in the southern Venice Lagoon,

provided, first, new information on the evolution, at high spatial and temporal

resolution, of the area over the last 2’000 years and, in addition, provided

evidences of salt-marsh system dynamic response to changes in sediment supply

during the last millennium.

For the first time in the case of the southern portion of the Venice Lagoon, a

detailed geochronological model for the lagoonal deposits has been furnished. The

study succession testifies an evolution from a palustrine freshwater environment

to a lagoonal environment, in which the depositional history started with

accumulation of peat deposits in a deltaic setting which progressively evolved into

a salt-marsh environment in the 14th century. The evolution of the salt-marsh

system is characterized by different accretion rates over time. Marsh aggradation,

stemmed out from both mud settling and organic accumulation, occurred in

parallel with the decrease in the salt-marsh extent and the tidal-flat expansion.

Indeed, where salt-marsh deposits were locally flooded and impacted by wind-

waves, wave erosion played a fundamental role, and a lag deposit developed. As a

consequence of the progressive water deepening, organic-rich mud accumulated

above the lag, originating tidal-flat deposits. The absence of salt-marsh deposits

below the lag and the tidal-flat deposits, highlights that the disappearance of salt

Conclusions

113

marshes in this area of the Venice Lagoon has to be ascribed to the lateral erosion

of their margins rather than to a progressive decrease in marsh elevations referred

to MSL and the following marsh drowning.

Finally, the evidence of a possible response of a salt-marsh system to

changes in sediment delivery rates provided by the repeated diversions of the

Brenta River in the southern portion of the Venice Lagoon is provided. The results

of a multidisciplinary approach based on sedimentological and elemental analyses

on the salt-marsh succession, were wedged with the detailed age model

previously presented for the area. The study reveals the existence of a temporal

lag between the presence of the Brenta River into the Lagoon and its detection in

salt-marsh deposits. The signature of the river along the marsh sequence appears

with finer inorganic deposition, decreasing of organic matter accumulation and

positive XRF peaks in elements which are consistent with the composition of the

lito-types which crop out along the Brenta River drainage basin.

115

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